Absolutely agree with JasonBay. The best tools in a mastering engineers arsenal are: a good room, reliable reference and a good pair of ears. If you haven’t already, I would do some research on mastering, be it reading (like JasonBay said), or preferably listening to examples. But this general advice won’t really help you get started will it?
It’s funny, a lot of sound experts like to give general advice, even though most of them started in very non-desirable circumstances. Just remember we all start out somewhere and that experience is the most valuable knowledge you can get, which is something these experts can’t just give you, unlike general advice.
That being said, while you research, get your feet wet. Try things out as much as you can, even if you don’t have the proper tools (yet).
In my opinion I would look at your budget, and see what can be done from there. Got enough money for decent monitors? Do it. You have space, likely to stay in it for a few years and can dedicate it to music related work? Start thinking about doing some sound treatment. These are good things in general, even if you don’t enjoy mastering, it will still help with any music work.
To directly answer your question. If you really need a plug-in, focus on getting a good stereo compressor. Waves makes great plug-ins, very professional quality, and they won’t break the bank. Hard to go wrong with a API 2500 or SSL 4000 Master Compressor. I say compression because it’s much more fickle to work with than EQ, especially when mastering! You’ll want to get a good handle on properly compressing stereo bands, and learning what the personalities of each compressor are. Once you do that, you can start chaining compressors, one after the other (which a lot of mastering engineers do). Also focus on doing as much as possible with as little as possible (less is more!). This does require you to really know and understand your tools through and through.
You can look into a good stereo EQ, but that’s less important, especially when you’re learning. In a proper mastering, with a proper mix, you will be boosting or cutting (large frequency regions) by 1 to 2 dB’s at most (if you even need to). So if you’re working on your own material, focus on having it sounding frequencially correct before it gets to the mastering stage. EQ-ing in mastering is generally just there to slightly enhance it, don’t think you’ll be making radical changes, or fixing problems with it.
In the end, just practice and get a feel for it. Mastering is completely different from mixing, and should be seen that way. A good exercise is to master a song quickly, only give yourself like 10-15min, take a break and master it again and compare. This will help you figure out what makes mastering sound better, and how to make better decisions based on what you hear. It’s also a good exercise because the pro’s master relatively quickly to begin with. If you decide to go through with mastering, look into buying a proper analog stereo compressor.
Hope this helps you get started!