I know of people who dont have any idea about mastering, compression\limiting or EQ who whack their tracks through T-Racks with dreadful results! and then they stick them up on digital download sites for people to buy!!

My advice would be to read up on compression\limiting first, then have a play around with whatever software your planning to use.
Do some trial mastering for a few weeks until you know what your doing.
Then after that atempt to master your tracks properly, try mixing them with other mastered tracks and get people who know what they are on about to give you feedback

Also less is more!!!
Try not to go overboard and avoid changing any EQ settings unless you have no choice (i'd advice going back and redoing the mixdown if theres EQ changes to be done)