Ok, you want the 10 cent answer.
Mix your tracks to stereo.
Put a EQ, Comp and Limiter on the buss in that order. Fiddle around with each until you like it.
Your best effort will no doubt be a waste of time because by the time you play it on a car stereo, home stereo, and computer speakers you will realize that your efforts were not all that transferable.
Mastering guys have very expensive stuff so they can make music transfer across many types of systems.
They also do things like meticulously space tracks to the beat of the previous track, arrange the order of songs and get it ready for duplication.
The best you can do mastering your own tunes is to make a nice recording for MOM, maybe a club, your best friend and your significant other.
Also, things you can't hear on cheaper systems that may be on your tracks i.e. excessive subsonics and hiss, are going to show up in a mastering lab. You will never even know they were there until people call you up telling them how your songs are ripping up their speakers, slightly exaggerated but can occur.