Originally Posted by contango

Depending on how hard your suspension is set, locking it out may make a significant difference. If you cycle up a hill you may notice the fork bouncing - if so it means some of the effort you're putting into turning the pedals is bouncing the fork rather than moving the bike forward. I've set my suspension to be fairly hard so it's not too much of an issue (I really should soften it up and use the lockout more)

If you put ergonomic grips on your handlebar you should find your hands don't hurt so much. Add bar ends as well and you'll have a few different hand positions you can use. Personally I find bar ends are also helpful when climbing and give me a slightly more aerodynamic position. It's possibly also worth checking out your saddle position - when my saddle was too far back I had the option of sitting right back (in which case my hands went numb) or sitting forward (which meant other more sensitive parts went numb). Since I slid the saddle forward both problems have all but disappeared.

If you're 13 the best bet is almost certainly going to be to add a few mods to your existing bike to see what you get on with. Then when you've outgrown this bike you can either get another mountain bike (in which case I'd hope your bar ends, ergo grips etc will simply transfer across to the new bike) or get a road bike then. It's easy to spend a lot of money chasing perfection when a much cheaper solution lets you try things out before you commit the big bucks.