A volley is when you hit the ball before it bounces. (Some people refer to a series of shots as a volley; that's really a rally.) A half-volley is when you hit the ball an instant after it bounces, usually just by holding your racquet near the ground and letting the ball bounce off.
Here are some things to keep in mind. Don't try to think of them all at the same time. Choose one that applies to you, concentrate on it until it becomes automatic, then move on to the next one.
If you're like most people, most of your good volleys are hit with your arm extended and bent at a comfortable angle, with the ball in your "strike zone" between waist and chest. Because you have less time to get into position for a volley, you have to make an extra effort to get into this position on every shot. Be prepared to bend your knees way down, or take a few quick steps toward or away from the ball, so that each volley is hit at basically the same position relative to you. Don't be embarrassed at making this extra effort for balls that are hit right to you.
Try to always volley the ball in front of you. On your backhand this may come naturally, because your arm is farther in front for that shot. Try to hit forehand volleys the same way, rather than letting the ball get parallel to your body.
There aren't really many general rules for overheads. There are a lot of different things that can go wrong with this shot, and each problem has its own solution that usually requires a lot of practice to make it a natural part of the stroke.
If you are off balance for most overheads, chances are you are not getting into position fast enough or are stopping too soon. Try to sprint into position, and be willing to take those last few steps forward or backward so that you're right under the ball.
Once you have a strong and reliable overhead, the big factor is placement. Rather than try to bash the ball hard to nowhere in particular, use the same tactics you would with any other attacking stroke: hit to the other player's weaker side, hit into the open court, and hit behind the opponent if they're already running to one side.
A "sitter" is a short shot that (so tradition goes) should be easy to put away. This expectation results in untold anguish as thousands of tennis players volley balls long or into the net.
If the ball is soft and high, don't let it drop. Sprint towards the net, even if you're already at the net. Get as close as you can without actually reaching over. Most people never get as close as they should for these shots. From this position, you can hit a hard shot downward into the court, or a soft angled shot.
If the ball is soft and low, don't overhit it. These shots may look easy but they're actually among the harder shots to hit. Expect to hit at least one more shot to win the point. The good news is: