Much like wrenching on cars, my plumbing skills are sorely lacking. That said, I have a leaking shower in our (very) old house. It looks a lot like the set up pictured below. The drip is coming out of the faucet, and no matter how hard I cranked down the 3 handles, it still drips. What do I do? Please use small words and lots of pictures! 
There should be a screw in the middle of the part you're turning. Remove that and take off the knob by pulling backwards. Remove the trim piece. You should see what looks like a nut right behind that (with the part you just unscrewed from sticking through). You should be able to carefully tighten that a little. If that doesn't work, you can pop out the guts and install new ones, by loosening that nut. Turn the water off before removal :)
It's really only a few rubber washers. You can rebuild it by replacing them. They also make a kit. Here is an exploded view. See the copper colored parts? Those are what you're fixing. Most likely the rubber washer at the end is toast.
Sounds like the packing (the adjustable seal between the shaft and valve body) is fine and it just needs new washers, shut the water off, open the valves to release the pressure, screw the two hot and cold valve assemblies out (leave the middle one) and take them to the local hardware store or lowesdepot and match up some new washers. Assembly is the reverse of disassembly. Locating the water shutoff is probably the hardest part.
http://www.doityourself.com/stry/how-to-replace-a-shower-faucet-stem
Remove the stems. Take them to a hardware store and match them with new ones. Replace and you're good.
Depending on how old your faucets are the screws at the end can be a bitch to get out. So I just replace them.
I have seen this done a number of times on "Ask This Old House" TV show. I would bet you can find similar info on "YouTube".
Harbor Freight shower valve socket set makes this job a lot easier. The hex is usually buried in the wall and the stems are usually long enough to keep you from using a regular deep well.
Don't forget to turn the water off. (;)
Step one.
Thanks everyone. I'll start with a trip to HF for the shower socket set. Then I'll shut off the water and commence with disassembly!
Give the brass seat a little fondle while you have the stems out. Its possible for them to erode over time. Two repairs are to buy a tool to cut the seats, or to screw them out and replace. If they feel smooth and even, don't worry about it.
Success! I first made a trip to Lowes, but they didn't have much in the way of selection. On to Home Depot. There, I was lucky enough to encounter Donna, the plumbing department manager. She spent a lot of time with me, and ensured I bought the right stems. I'm glad she was there, as I almost bought course thread stems when my old ones were fine. Got the water shut off and made the swap. No more leak!
Streetwiseguy- I'm happy to hear that. The kit I bought came with new seats, but the old ones seemed REALLY stuck so I decided to leave them alone. 
What? One trip? Buy a Lottery ticket, average plumbing consists of 3 - 5 runs.
You're very smart or very lucky. 
they are really simple to fix. Worst that can happen is it leaks
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