http://www.denialville.com/bathtub-faucets-parts/
Should try to fix a Leaky Bathtub Faucet by myself or call a plumber?
I've never done it before. I can read directions, but I'm afraid I might break something. If I do it, I'll just replace the entire faucet instead of sorting through the intricate parts.
Or should I call a plumber instead?
Fix it yourself. Go to this site and follow the directions
http://www.doityourself.com/stry/h2repairvalve
It's very simple and you will only spend a couple bucks on a rubber washer instead of hundreds on a plumber. Just turn off the water first, so if something unexpected happens you don't end up with a flood. Use the correct tools, and have some WD-40 to loosen anything that's rusted. Replacing the faucet is just as much work as fixing the leak, except it will cost you more money.
Kohler - Sterling Whirlpool Bath Products