listen to the music you like the most. Do some research on the band, and find out what their influences are, and have a listen to them.
You already have a guitar, which makes my next point irrelevant;
If you are looking to get your first instrument, make sure its inexpensive, but comfortable. Theres no point in getting an instrument if it doesnt feel nice to play. Squier make some really nice cheap guitars. I had one for 3 years before i got my fender.
Its a bit harder with guitar, because the authorities are getting anal about guitar tab sites, but see if you can find some tabs for guitar. A good site is mxtabs.net for guitar, and i like bassmasta.net for bass tabs. Tabs are sheet music created by another guitarist based on what s/he thinks the song goes like. More often than not, they are slightly flawed, but thats good, because it gives you room to improvise.
find some good mates who play the other rock instruments (bass and drums) and make a band. This will get you motivated to play, and will stop you from becoming whats known as a "bedroom guitarist"; someone who plays just for themselves, and sometimes ends up becoming whats also known in the music industry as a 'wanker'

A lot of bands form in their high school years; Silverchair is one example. They wrote the song "Tomorrow" when they were in their early teens, and that song is still among the most requested songs ever on Triple J.
Last point, just have fun. Thats all that music should be about. If you aren't having fun with it, kick out the band member which is the cause of the problem.
If you make a band, create a
PureVolume and a myspace music site. They are both free, and are viewed a lot. Be sure to give us the first sample of your music!
Enjoi
Lee from JjR