Well typically you should actually get a mastering engineer to master. Some people say its more of an art than a skill. That being said you can get the izotope ozone plugin which is a general mastering plugin. Thats my vote.
You can't 'master' without years of experience and several thousand pounds' worth of pro gear.

However, you can 'pseudo-master' home recordings, provided you've got really good monitors. Izotope Ozone is really good for enhancing your tracks, and AAMS is a reasonable automatic mastering system - it's no match for a good engineer, but worth a try if you want to get a home recording on radio,TV or whatever and haven't got the cash to send it to a specialist...
what exactly is it you do when you do mastering? Is it just fiddeling with EQs and mixing volumes of everything?
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Ok pretty much is finalising a mix. Once you've mixed everything you bounce it down to a stereo track. You then add compression, final eq, a mastering limiter and stereo widening. The reason you get someone else to do it is because, you just mixed these songs, all your gonna do is make it louder... thats it.

Mastering takes place AFTER the mixing process, its what you do before you put songs on cd.