yep, I second what steve's saying.
Really, just start by playing around with what you've got. That way you start to learn the process, without spending any new cash.
I'll also suggest, for your first times, to pick nights with a full (or near full) moon. The stars get muted. But it's easier to get brighter images at lower ISO's. I also just use the self timer mode to trigger the shutter. That way you don't jostle the camera when you press the button.
So the pics I posted above were taken with either a Nikon D40, or a rebel Xti.
But the first camera I started playing with was an olympus Point and shoot that only had a 16 second max shutter timer.

Which would have worked better, if the camera didn't have such poor "higher" ISO performance.
(pic above was ISO 160) Click on that pic and you will see loads of red white and blue spots.
This was just one of the shortcomings of that camera.
It's performance was tolerable, however, if I kept the ISO to 80 or below. (like this pic)
So.
Since I quickly hit the limit of my camera... I learned a new technique... "painting" with light (and for the first time, got to play with a DSLR, and a 30sec. shutter. ISO 100)
So I guess what I'm really trying to say is, "Just get out there and start shooting". You just might be surprized at how much you can actually do.
