Sports photography is pretty tough, especially if you don't know the sport. I found myself photographing football recently, and I have a really hard time even finding the ball some of the times. In addition, AF speeds, while they are fast on my camera+lens, aren't quite as fast as a running quarterback.
When it comes to sports, there is one major thing that everybody can do to get a better picture - sacrifice added grain for reduction of motion blur. You can fix grain, but you can (very, very, VERY) rarely correct motion blur. There are numerous photoshop plugins to reduce noise.
While almost every sport (with the exception of bowling and golf, I believe) allows you to use a flash - I would recommend trying to take your photos without them. There are a couple reasons for this:
If you are shooting HS sports, parents can get very angry at you for using a flash - and will blame their child's horrible play on the fact that YOU distracted them (yes you, and you alone)
Recharge rates for flashes are not very high, for sports, I've found that it's best to not be limited by a recharging flash. In addition, the harsh non-diffused lighting is distracting, and casts odd shadows on subjects in some cases.
It is more likely than not that your flash will not even reach the player enough to light them well enough to get a good shot.
Getting a great sports shot is, to me, all about expression. You can't tell what someone's expression is if you're shooting their back with the ball in front of them. For this reason, I would suggest being in a corner of the field rather than in the center. Alternate the corners when you can, or depending on the amount of time the ball is on either side. By doing this, you can get shots of players coming TOWARD you, and thus hopefully some stellar shots of their face when they make their goal.
I hope this helps, and good luck