Well, its pretty obvious, but if you want to build arm strength, you must work the muscles in your arms...heres some basic to intermediate workouts for arm strength though:
*my favorite is the preacher curl because there is no real way of cheating it. it has good isolation on the biceps, and there is a large negative phase in the lift as well.
-tricep cable extensions
-close grip bench press
*my favorite is the tricep extension on a bench. lay down on your back, and make a triangle with your thumbs and index fingers and lift the weight from behind your head.
practically any of these workouts can be performed with a barbell, dumbell, or even bent barbells(meant for curls). if you need help with any of those exercises, check out bodybuilding.com.
hope it helps
BTW, 14yrs old is pretty early to start weight training, so be careful, and don't stress your muscles too much. stick to about your 70-80%MRP. just figured i'd throw that disclaimer out there to you.