Figure Out Your Needs
Maybe you're a road biker looking for a new adventure. Maybe you haven't been on a bike since you were 15, or perhaps you're a mountain veteran in search of a new frame. No matter why you're considering mountain biking as a hobby, figure what your needs are. Will you be on the mountain once a week or once a year? Are you just getting off pavement, or do you plan on slaloming through trees? Do your research with professional and niche publications, gathering as much information as possible.
If you're taking the plunge and making a purchase, you need to figure out your budget. A mountain bike frame will run anywhere from $600 to $1000. Spend any less, and the manufacturer might be cutting corners--and making unsafe frames. Any more, and you're pushing into professional frame territory, which might be beyond your needs--or be at a shop that price gouges. Figure out why you're purchasing, or even how much you like mountain biking. This will help you set your budget. And remember--this can be the price for the frame. Things like tires, wheels and seats cost extra.
Heading to a specialized store is the way to go, rather going to the local superstore and purchasing a bike simply marked "mountain bike." The dealer can help you find your right bike in your right budget. They will listen to your needs and help you ascertain whether you need a rough-and-tumble frame, or a lighter weight, dexterous bike. Your riding style is important in figuring out what frame is for you, and a dealer can also fit you for your frame, making for a safer ride.
Steel vs. Alloy
Weight and durability are the key words when purchasing a mountain bike. You'll want a frame that is heavy enough to not submit to the elements, but light enough to pedal over hills and bumps. Durability is another obvious choice--if you crash, you want your $800 investment to survive. A well-made alloy bike will be costly but will often be the right balance between weight and durability. But be wary--if you're not strong enough to bring any bike up the mountain, the bike you ride down on won't be durable enough to survive a crash.
Test Drive, Test Drive, Test Drive
Even if the dealer won't let you take their bikes up on the hill, get on and ride them. Up and down the aisles of the store, in the parking lot or on the sidewalk--be sure to get in the saddle and pedal your heart out. Your dealer can observe your stance and help size you for your frame. But be sure to follow your own comfort levels. Even if your dealer says you're on the perfect bike, if it doesn't feel right, don't buy. Confidence is one of the things you need to stay safe on the mountainside.
What to Choose
In the end...generally speaking, if you're a serious rider with the skill to handle what the mountain throws at you, try an alloy frame. But if you're just starting out, you might need some heft in your bike to keep you in the saddle. Opt for steel.