So you've been wondering about getting a mountain bike and thinking what it might be like to try some "real "mountain biking.
First up to consider is your body size and its' relation to the bikes fit and it's setup for you.
Bike Fitting & Setup for Safe, Efficient, And Fun Mountain Biking.
Please keep in mind that there will be a number of similarities in bike sizing and setup that may be applicable to just riding a regular road bike, hybrid, etc. There are however enough differences to seriously consider mountain biking as its own very separate sport.
There's a lot of small factors to consider for your first x-country mountain bike and ride- frame sizing, three seat adjustments, footwear/pedals, handle bars/stem angle. brake levers
Frame choices: small- 13"-16" (5'-5' 2"height), Medium 16-18" (5'2-5"8"), Large 5'8" up 19"-up. Straddling over center bar with feet on ground you should have at least an 1" or more clearance under you & top tube(frame)
Seat adjustments: height- when sitting on seat take a down pedal stroke and your leg should be almost entirely extended (only very little knee bend) when foot is on pedal on ground.
Seat angle: it should be completely horizontal to level ground or to a level center top tube.
Seat distance from handlebars. Arms should be almost completely extended(only very little or no elbow bend) when on seat and grasping bars.
It should be noted that loosening the large allen screw behind the seat will permit the seat to slide and the angle to be changed easily on the seat rail.
Footwear/pedals: If you're a novice and are not experienced with clip-less shoes & pedals, start with a platform pedal with straps and toe clips
This is for safety. You need restraint, something to help keep your feet on the pedal when you occasionally encounter bumps and more often then you think you will be standing on those pedals not sitting on the seat. The preferred and safe footwear for toe clip and strap system is any treadless sneaker or shoe, basketball ,tennis etc. No trail jogging or hiking shoes.
Handlebars: The newer bikes often have riser bars. These put you in an upright position which is considerably more comfortable on the trail and offers you far better vision and bike maneuverability. Your handlebar angle comes from the stem which is (rise) of your handlebar this angle again is adjusted often by changing your stem and also can put you in a more or less upright position.
Brake Levers: Hand size, finger length, is the important factor here to consider. If you have difficulty reaching the levers easily there's a tiny allen screw that can be turned to bring the levers in making them much easier to reach. Remember, difficulty in fine tuning any of your bike's adjustments can be easily accomplished at any reliable shop.
Helmet: Most important safety feature. Never ride without one and straps should be adjusted so helmet is snug, not tight, and doesn't move when you shake your head front to back or sideways.