Re: difficulty with serve
When you need to work on something go to the basics:
1. Make sure your grip is the "hammer" grip. Act is if the side frame of the racket to "hammer" things. This is the most common grip to use.
2. Have a friend/coach watch your serve toss. Many players have bad serves because of bad toss placement. Make sure the ball is right in front of you and doesn't get too high or low. When you toss the ball, you don't want it to land more than half a foot in front of you.
3. If you are right handed, you want your plant foot (left) to be facing toward the net post on your right. If you are one of those people who pulls their non-plant foot (right) towards their plant foot while serving (like Llyeton Hewitt), make sure the non-plant foot does not surpass the plant foot or else you will end up trying to bend ackwardly (and look ackward).
4. Keep your tossing hand always up in the air until you start your swinging motion. Do not bring your tossing hand down too fast; you want to pull it down at about the same speed you are swining your other hand to serve.
5. Always keep your head up after the ball is in the air. You bring your head down only after you have made full contact with the ball. Do not bring your head down as soon as you hit the ball however. Many players do this and it ends up with them bringing their head down too soon and the ball going into the net. Wait after the racket has pass through the hitting area.
6. Bend your knees during your toss and try to time them to start and finish at the same time. Kick your knees during the swinging and again, try to time it together. Timing is important when bending your knees because you don't want things not to work to the full extent. These things may occur if timing is not correct: If you bend your knees too fast, your toss may become disoriented. If you bend your knees too slow, you might have less time to react to the toss, making the ball drop too soon. If you kick too fast or slow you won't get all the power stored from your legs.
7. When practicing the serve, and especially warming up the serve, just make sure you are making 100% contact. Do not try slices, kicks, etc, just make total contact, even if that means you have to swing slow.
Best tip is to watch a professional though. Mimicing other players is a great self-practice method.