For the most part, showers and faucets are the same.  One's horizontal and one's vertical, and the parts in a shower are bigger.  Otherwise, the same principles and procedures apply.
 

LOW WATER PRESSURE

If it's gotten lower over time, it's the shower head.  Unscrew it from the shower pipe, then take it apart and find what's clogged.  It might be like an aerator in that it'll have a water flow constrictor whose tiny holes are filled with sand.  Take a pin or needle and clean them out.

If it's just low in general, it probably has a water constrictor in it.  Remove it from the pipe and look for a (usually white) piece of plastic with a small hole in it restricting the incoming water.  Remove it however you can.  If it's permanent, drill the bastard out.

 
DRIPPING

Here's the standard routine.  Your mileage might vary:

Determine which side is dripping.  It's usually the hot side.  If the cold side seems to be working perfectly, leave it alone.

  

Turn off the house water via the outside valve.

  

Pry the little plastic cover off the top of the knob.  Unscrew the bolt.

  

If the shower's really ancient and crusty and has metal handles, you'll probably need to pry on it from below with a couple of screwdrivers.  There's also a special faucet handle-pulling wrench at the store.  Rock it back and forth in all directions and then give it a try.

  

Unscrew the valve with the crescent wrench.  Some valve stems are buried deeper in the wall and need a deep-well socket.  A regular deep-well socket probably won't work, but you can try.  If not, head to the plumbing department and buy one of the deep-well sets made for plumbing.

  

Unscrew the faucet washer from the end of the valve.  Replace it, then reverse the above process.

 

The problem with dripping faucets and showers is usually the faucet washer, but not always.  If you replace the washer and it's dripping again in 6 months, it's because the little brass piece that the washer is pressing against has a rough edge and is grinding it down.  If it's corroded enough that there's an actual cut in the valve seat, just replacing the washer won't stop the drip in the first place.

There's a fairly cheap tool at the hardware store for removing valve seats, and there are only a few sizes of them, so pick up a new seat and you're good to go.

In older showers, the valve seats aren't removable.  The store sells a 'valve seat grinding tool' that you can either operate by hand or use with a drill gun and this'll grind the valve seat down until it's smooth again.
 

REPLACING

Actually replacing the whole valve assembly is usually above the average person's head, as it often involves soldering brass pipes with a propane torch, and that's not to mention replacing the tile afterward, or whatever the wall was made of.  You can put on new handles and back plates, but best leave the behind-the-wall stuff to the pros.