Many mid-range bikes are made of aluminum. Carbon-fiber bikes are light and ride well, but they aren't cheap. For mountain bikes, allow more room-about four inches. The seat should be adjusted so that your legs are about 90 percent straight when your feet are at the bottom of the pedal stroke while sitting in the saddle. Your knees should never lock, and you should never feel like you're reaching with your legs as you pedal.
Some seats also adjust forward and back and tilt up and down.
Don't worry about these adjustments for now unless the seat is uncomfortable. If that's the case, head to a bike shop for a set-up, which shouldn't cost much, if anything. You want to be able to reach the handlebars without overextending your arms or hunching your shoulders up around your ears. Your shoulders should be relaxed, elbows bent and back straight. The most important cycling necessity, besides the bike, is the helmet.