Angie, there are several different types of internal faucet configurations and it might be tough to cover all the potentials, but I'll try. The normal places that leaks occur are around the stem packing, which results in water running down the outside of your wall, but sometimes inside the wall, and at the washer or disk type cartridge that results in a drip at the faucet.
You begin the job the same way for either. TURN THE WATER OFF to the faucet. There will normally be shutoff valves in the piping under the tub. They are usually under the tub in the basement for a first floor bathroom, but sometimes you get to them through a removeable panel on the other side of the wall at the end of the tub where the faucet is. Turn the valve handles clockwise from the handle face side to close.
Now open the faucets slightly and remove the faucet handles. Usually, there will be a screw visible on the face of the handle but sometimes there will be a cap with H or C on it and the screw is under the cap. Gently pry off the cap if yours has one and remove the screw for each handle. PUT THE HANDLES AND SCREWS SOMEWHERE SAFE--THEY LOVE TO ROLL AWAY.
If your faucet is leaking around the stem, the leak should now be easily visible. First, tighten the stem packing nut a little with a crescent wrench if possible but you will probably need one wrench of a 5 piece set of wrenches that you get at a home center for this. You want to just snug the nut up. Turning it too much will make the faucet handle hard or impossible to turn and could damage the packing so it will leak more or you will permanently hurt the parts.
Because of the probable length of this, I'm going to do it in several installments. This ends part 1.