Use. MTBs are available in any kind, type and price from 100 US$ to 5,000 US$ or more. Therefore it is very important that you try to figure out how you will use your bike. Mainly in the city on paved road? Then buy a low-cost city bike with kickstand, fenders, lights and reflectors. On city streets or just outside on large and flat country roads? Then a basic rigid MTB is perfect for you, don't waste your money on a superlight full-suspended bike! Mainly off-road, for long tours but on not-too-steep dirt roads? Then a medium level MTB (LX shifters) is okay, maybe a front-suspended model depending on your budget. If you really think you will use your mountain bike as its name implies, i.e., up steep and technical trails, down in the middle of woods, grass, rocks, critical steep single-track, then you have to buy a good bike, reliable, light enough for climbing, with at least a good front suspension fork. Of course choices depend on your budget as well.

Price. My suggestion is to avoid spending too much money for your first bike. If you are a beginner it will be very difficult to choose the right bike, you risk buying a wrong and very expensive bike and you cannot know exactly what you need or you like more. A very stiff frame? A long-travel fork? A pure downhill bike? How can you know? So stay on a plain model, maybe even a used bike, get some practice. You can upgrade your bike later or sell it and buy the real bike of your life.

Brand. One of the most common question is: "Every store I visit, it's the same story: they all say their bikes are the best! How can I choose then?". First of all keep in mind that usually the manufacturer is just assembling a bike using standard parts available on the market (mainly from Shimano). Some manufacturers just assemble carefully selected components, some design a frame geometry but have someone else build the frame using standard tubes, some actually weld the frame but they use standard Tange, Columbus, Easton or other-brand tubes. Very few manufacturers are so large, well-established and with a real R&D Department to manufacture a complete frame from scratch. For example Cannondale for aluminum, Specialized for the metal-matrix and Merlin for titanium. In any case no one manufactures his own shifting system. So if you are familiar with the different components you can choose a bike without even looking at the brand. Add to this that the frame is not so important for a beginner because the difference in weight is the only practical fact that everyone will note. Geometry, angles, stiffness require experience to be really appreciated. But I understand that a beginner cannot be an expert and cannot distinguish between an Alivio, an XT, or whatever. And this is why I recommend considering a very well-known brand (not because well-known brands are necessarily the best). If you do that, chances are that you will get a lower price, a very broad choice of models and a more serious dealer, but this is not a fixed rule, it's just a suggestion. Please do not misunderstand me: some small local companies can be much better and more customer oriented than big brands, but a beginner can't usually find this out , that's the only reason why I say: go to a big-name manufacturer. But if you have an experienced friend that can help you, don't be afraid to buy a no-name bike.

Dealer. If you are a beginner, you are not an expert. So do not underestimate the importance of your dealer. He or she can help you, understand your needs, find out the right frame size (the most common problem) and service your bike. On the other hand s/he could also sell you a bad and even expensive bike, an old model he cannot sell, a wrong frame size (because it's the only one he has on stock). A serious dealer will probably have good and reliable brands in his store and will also help you after selling the bike. So do not look only for the lowest price, look for the right dealer too. Test him to see if he is serious and competent: ask tons of question and notice his answers. Check if he is biased toward a specific brand. If he lets you speak and tries to understand your real needs, if he accepts to service the bike free of charge after one month (spokes, cables, nuts and so on need to be tightened after a month or so), if he allows you to test a bike, if he can explain to you the differences between various models using clear terms you can understand. Do not be shy, you are the customer and you have the right to see and ask everything.

Frame material. The frame is a very important component, because you cannot upgrade or modify it and it lasts forever, so changing a frame usually means changing the entire bike. The most common question about the frame type are related to the material. I will try to answer some typical questions. First of all, for a low-medium level bike, aluminum or steel are both perfect materials. But remember that there are many different types of very bad steel, the one used for years for common city bikes, very robust but also very heavy. On the other hand, you will not find very bad aluminum frames, NOT because aluminum is superior, but simply because it was created as a new product for high-quality bikes (e.g. mountain bikes) and NOT with the aim to substitute steel for very poor quality street bikes. So if you choose an aluminum frame you can be almost sure it's of a good quality, while with steel you just have to ask which steel it is. It's not so complex, just follow these two rules: avoid if possible HI-TEN steel, which is the worst for mountain bikes (I mean it's heavy but if you have a very small budget it will work fine for twenty years!). Second rule: double butted (or even triple butted) frames are of better quality because they have tubes with different thickness to decrease weight and keep resistance high. Another sign of good quality is TIG welding (which is absolutely necessary for aluminum due to the difficulty of welding it). I repeat that, generally speaking, aluminum is NOT better than steel, so don't be fooled by your local salesperson. Still I can confirm that an aluminum frame is usually of good quality due to the difficulties involved in manufacturing and welding it. Some other differences are the following. Steel is flexible, so if you have no suspensions, steel will offer you a smoother ride than aluminum. Aluminum is difficult to weld, so be careful with extremely low price no-brand frames, they could break! Steel can rust, aluminum won't.

Frame size. It is probably the worst nightmare for many beginners. This also depends on the fact that for road bikes the frame size is well known, depends on the physical dimensions of the biker's body and can be established with a fixed table. For a mountain bike this is not the case. There are no tables. Moreover the frame geometry can vary a lot from manufacturer to manufacturer so if you buy a 20", when you change brand maybe you need a 18" (usually the frame size is given as the length of the seat tube). Add to this that mountain biking is a much more complete and varied sport than road biking, so the right frame for climbing is not the best for downhill. In other words the right frame is a compromise depending not only on the biker's body but also on the MTB use and the personal preferences of the biker. So, instead of becoming crazy, let's consider a way to choose a decent frame. Of course we are talking of your first bike, so the choice is not too complex. I usually suggest this practical approach which is not THE solution, but can help you.
TEST 1) Move one pedal to its lowest position. Then raise the saddle so that when you are seated on it and put your foot on that pedal your leg is almost (but NOT completely) extended: that is the correct saddle height. You will therefore touch the ground with your toes only. Then dismount and take a look at the handlebar height (which is usually fixed). A correct height is some centimeters below the level of the saddle or at the same level. If the handlebar is higher, than the frame is too big, which means you will have to stay too stretched horizontally with a difficult reach of brakes and pain in your shoulder. If the handlebar is 10 or more cm lower, then the frame is too small.
TEST 2) While moving around trying to go with your belly on the saddle and your butt on the rear wheel, almost touching it. That's the correct position for extreme limit downhill. If you cannot reach this position probably the handlebar is too far, that is the frame is too big.
TEST 3) Try pedaling riding in circles as much as you can and notice if your knees are too close to the handlebar, in this case the frame is too small.

Brakes and drivetrain. After the frame what should you check? The moving parts, the one that are more likely to drive you crazy, to break, to squeal, to fail. So we are talking of: shifting system (rear and front derailleur and shifters), chain, cogset, crankset and brakes. You will hate a lightweight bike with a bad derailleur but you will love a heavy, old MTB with a good shifting system. Shimano's groups XTR, XT, LX e STX are all very good, reliable and light. If you have a reduced budget, Alivio is good too, but don't get anything less if you want a reliable and enjoyable bike. Regarding brakes, now almost all new bikes are equipped with V-brake or similar brakes. They are usually good enough for a beginner.

Shifters. What about Grip Shift shifters? They work, they are more reliable and simple but probably less intuitive, less cool and easy to use. For a beginner both shifters are okay if the quality is not too low.

Mixed components. A very good idea is to mount different components. This is becoming more and more common. The best quality for the most critical part, that is the real derailleur, then a little worse for front derailleur, shifters and maybe hubs and a low quality for cranks, that are after all two pieces of metal. For example you can find: rear derailleur XT, front LX, cranks STX. Be sure to check the name of the components, a rear XT does NOT mean all the components are from the XT group.

Sunspension. Suspension forks today are usually reliable, safe and with reduced maintenance. They are also surely useful, will increase riding comfort, forgive your driving error, relieve stress on the wheels. So if you can afford a suspension fork you will be happy with it. BUT do not buy very low cost suspension forks (less than 200 US$, just to give you an idea, but prices change widely all over the world and over time). Basic forks will increase the price of your bike, will weigh a ton and have such a limited travel as to act like a rigid fork, so in the end you will have weight and cost disadvantages but no benefit at all! Don't buy them. Always inquire about the weight and the travel. Keep in mind that a fork weighs like a complete frame and often even more. Regarding brands, the most common are Rock Shox, Answer-Manitou, Marzocchi, RST, Suntour. All of these are of reasonable quality.

Used bike. A used bike can be a very good deal. You can have a better MTB for a lower price. So, especially if you are in doubt about your love for MTB, consider also this possibility. You can buy used bike also at your local dealer. In any case remember that the risk is higher. I suggest you check carefully every part subject to wearing or breaking. Check particularly the tire thread, the brake shoes, the chainring teeth, the chain and the cables. Also the bottom bracket should be checked but to do it you must dismount it, a long and not easy operation. So at least do the following: grab a crank firmly and shook it to see if there is even the slightest play. If this is the case the bottom bracket must probably be replaced. I suggest to check the used bike with an expert rider. And, in any case, jump on it and pedal around.