In general the mastering process is merely a second set of ears. Somebody who is not emotionally attached to the project and who can tweak a good EQ and a good compressor. If necessary, that somebody may be able to make your music shine quite a bit more. May even be able to feel that you can touch the sound. But only if necessary!! And who decides that?!
Compression helps bring out those elements in your music that tend to hide. It is quite a complicated balance to use compression so I suggest you go online and look for info about that.
Good mastering is done on dedicated analog equipment and a Rolls Royce is normally cheaper. There is lots of software out there that can do your compression and cheap too. (cool edit, cubase etc) but keep in mind that the sounds in your keyboard are already compressed for max performance so that further compression will squeeze the sound and ruin your final picture.