This is a bigger question and you will benefit by discussing this with multiple sources, including a local bike shop. At a minimum, you must have a mountain bike with 2 inch or wider knobby tires. Taller riders typically ride a 29 inch wheel size because of the rolling advantages of this tire size. Shorter riders, such as those in middle school, are typically on a smaller wheel size (26 or 27.5 inches).
Some riders choose to ride full suspension bikes. While the advantage is a more technically capable bike that offers better traction through rough terrain and turns, the downside is added cost, weight, extra maintenance.
If you can afford it a full suspension bike is typically the best way to go. If you want a fast, light bike for the least money possible, then a hardtail (front suspension and no rear suspension) may be best. There is no perfect bike and remember that bike fit, geometry, and weight are all important factors.
Happy shopping!
