Mastering is an art.
It's really only eq and compression, but you could spend most of your life learning it.
I'm not really great at it, even though i have mastered a couple of things, "divided by lightning" by phiiliip.
link
the animals issue of
K48
and a few other project you're even less likely to have ever heard about.
anyway, one thing i can say is this, with mastering, less really is more.
a lot of people who are first starting out go all crazy with extreme multiband compression and lots of eq. Not a good idea. Just some light compression to get your percieved loudness up (the main goal of mastering) and maybe a touch of subtractive eq.
For somebody who is just starting out, just use something like waves L1 or the free clone (w1). Set the threshold just low enough to get some gain reduction going on, but not so much that you crush the dynamics altogether. This leaves a lot to be desired after you have some experience, but when your just starting out, this is a good way to get some more percieved loudness without screwing anything up.
as far as learning how to use compression, just experiment with it a lot. Don't be afraid to do things you aren't supposed to. Trust your ears. (i'm assuming you know what a compressor does in theory, if not, i would think there are plenty of resources out there on the web, get googleing)
.lm.