Hitting down the middle to your opponents is the key to winning points. It can create a sense of confusion for your opposition and most of the time will either win you the point outright or put your team in a viable winning position.
Rush the net behind a good serve. This puts pressure on your opponents and may cause them to take their eyes off the ball for a second, which is enough to draw out an error from them sometimes. The first volley is important and will be effected by how well you serve.
Work on having a reliable second serve. If your first serve misses, you will always have "old faithful" in your second delivery. This is especially important in doubles where the aim is to get as many serves into play. You can't win points if they don't go in.
The person at net when his team is serving, is responsible for pressuring the opposition. Stand about two racket lengths from the net, and keep to the middle between the inside tramline and the centre service line. Keep the opponents guessing as to whether or not you will cross to poach. Mind the tramlines too. You opposition will sometimes try to "keep you honest."
Be weary of the net person. A short reply can often lead to the opposition knocking it off with one swoop. Throw in a few lobs. Go down the line a few times when the net person seems to keep inching away from it.
Mind your footwork up at the net. It is important to stay on your toes at all times, but especially when up at the net where your movement has to work in time with your reflexes. In other words, be alert.
Always make verbal communication. Call a shot when it is clearly yours. To avoid confusion, only call "Mine," and never "Yours." This is a personal thing of mine. Calling "Yours" can lead to confusion or put your partner under pressure to reach for a ball that he may not be confident of reaching. Encourage your partner if he is not playing well, but also when he is
playing well. Use words to effect that you are a team, not two islands on a court. Lots of "We" and "Let's" phrases, and nothing to single him out as this adds pressure.
Lastly, work as a team in synch. When one person has verbally decided to chase a lob, both of you should run back together. Ensure that there is some sort of mutual agreement to rush the net at every viable opportunity. Don't forget to switch sides if your partner has decided to make a sudden chase for a shot behind you that you yourself were not able to reach.
I hope all of that makes sense. It's fundamental stuff I learned when I was about 12. It really wasn't until I was 15 that I realized that these things actually work if you try them. None of that one-up-one-back, keep-to-your-side stuff. Ugh.
Oh yeah, and congratulations on making the tennis team selesfan1