Here's a quick little one minute guide to some footwork techniques. It doesn't cover everything because A) I don't have time and B) I can't think of everything.
1. The split step. What you do every time the other guy hits the ball. You want to time your hop, which can be off both feet or one, so you are at the top of the hop when the other guy is at his contact point. By the time you land, you should know where you have to go. If you have to go to the right, land on your left leg and push off. If you have to go to the left, land on your right leg and push off.
2. The steps in between the split step and your setup for your shot are kind of intermediate steps. They should be fairly small, but don't overdo the smallness. Oftentimes, you'll want to take bigger steps if the ball is farther away. Line the ball up with your back leg.
3. When you are about to hit the ball, you have to get your back leg down first. The back leg is the important one, the leg you plant so you can get a strong foundation. If you have time, you can always step with your front leg just before or during your shot. This goes for volleys as well as groundstrokes. Reaction volleys are often hit from open stances, whereas volleys where you have more time often involve stepping with the front leg during the hit, not before the hit.
4. After you hit the ball, you have to recover. Pivot off your back leg, which should still be planted on the ground. If you leave the ground during your shot, make sure to get the back leg back down. Often, the back leg will the outside leg. Recover to the spot where your opponent's reply is most likely to come. A bit of a crossover step with your outside leg crossing over in front of your inside leg may be necessary. Simple court geometry. If you're at net, you want to be positioned more to cover the down the line pass, whereas if you're at the baseline, you want to be ready to cover the crosscourt shot more.
5. Once you've recovered, just hang out there with your weight on the balls of your feet until your opponent is about to strike the ball. At that point, it's time to hit the split step again.
Rinse and repeat. And when you serve, you've got to split step when your opponent returns the ball. When you return, you have to split step when your opponent serves the ball.