Choices can be overwhelming when you set out to buy a mountain bike. Don't get talked into buying more than you need. Start with the basics: an aluminum- or steel-frame bicycle with wide, knobby tires and lots of gears ― 27 is good.
A. Bike. Bikes are made from aluminum, steel, carbon fiber, or titanium. Aluminum is the most popular material. Titanium and carbon fiber make for strong, lightweight frames, but they are pricey. Don't buy a hybrid bicycle ― they aren't really made for off-road riding. Good fit is key: Make sure the salesperson sizes you. In general, you want two inches of clearance between your body and the bike when you are standing astride it. Prices for a decent bike start at $600; you can get an excellent bike for less than $1000.
B. Suspension. Most mountain bikes have front suspension ― shocks that cushion front wheel bumps. Many upscale bikes have front and rear, or full, suspension for a cushier ride. But for a first mountain bike, that isn't necessary ― and it usually costs a lot.
C. Shoes. Shoes should have stiff, flat soles ― running shoes won't work, but lightweight hiking shoes can. It's important that your shoes are comfortable, can get dirty, and aren't too heavy. Eventually you'll want to get special shoes to attach to clipless pedals, which can improve efficiency. ("Clipless" is a misleading term; the shoes clip directly to the pedal.) But avoid them until you're very comfortable on the trail, because they can be tricky to get out of at first. Bike shoes cost about $80.
D. Helmet. Fit is important: Sizes or adjustable pads help to ensure that the helmet is snug on your crown. The chin strap should not gap. Helmets cost from $50 to $200; the more expensive ones are lighter and have better ventilation.
E. Shorts. No matter how silly you feel, get padded shorts. They cost about $60, and you will appreciate every penny you paid after an hour of riding. If you're not into skintight fashion, baggier padded shorts are available. (And no, nothing should come between you and the shorts.)