TexRoadkill has some good suggestions. Some will say there are no rules, but there really are. Listen to professionally recorded/mixed cds and you will find most of them use the same basic panning techniques. So my first suggestion is to listen to some music you really like, or that is similar to the music you are tracking. Where did they place the kick, snare, rhythm guitar, lead guitar, vocals, background vocals, keys, etc. You will begin to see patterns and this will also help you to 'visualize' your mixes better. Some of the things that are very common are, kick, snare, bass, vocals center. Background vocals panned equally left and right. Rhythm guitar panned many times hard panned. And a ton of variations of instrument panning with various effects. But there are definite patterns that will give you a great startign point.
One thing TexRoadkill said that is key is to keep your mix balanced. There shouldn't be a whole lot going on on one side and not the other, so if you pan your rhythm guitar right, consider panning that piano or horn part left. If the guitar is the only thing playing, double it and pan them away from each other but try using a delay on one side to give them a sense of seperation. Always keeping the left/right relationship in mind.
I have without doubt learned more from listening to what others have done than by reading articles on the subject.
Have a blast and good luck.