it entirely depends on your music. first question: what sounds good? If you've heard something that you're trying to recreate, or creating something new to match what you hear in your head...what's missing? Get that down first. There's no science to any of this.
Once your tracks are down, you can add some effects if you haven't already...reverb, eq, compression, etc. Get your panning where you want it, and levels set.
Then, once you've got things down to a final stereo mix, you're presumably going to record back into a PC (or tape), then "master" it from there. Most mastering doesn't really happen until you put an album together, since you're trying to make the album sound coherent...a "whole" rather than seperate songs.
in the mean time (ie- when you've only got a song or two), you can just tweak EQ's, normalize, and/or send the final cut through a compression.....preferably multi-band compressor especially with drum-heavy tracks, it keeps the thumps from squelching your melody. Point is to get the levels up (use the most dynamic range you can) and the details heard (that's what compressers are for). If you need more info than that, you'd be better off picking up a copy of MIX magazine to at least earn the lingo (makes it easy when you know what you're asking for), or do some searching on mastering or finalizing.
