How do I fix a faucet (I'm new at this do-it-yourself stuff)?

My shower faucet doesn't leak when the water is off, but when I turn it on (for a shower) some of the water is going down to where the bathtub spout is instead of through the shower head. I have the "shower lever" pulled up all the way, but a lot of water is still going down through the bathtub faucet (where it comes out for a bath.) I can't find any good youtubes or instructions for this, so I need you guys! Thanks :)