I'd recommend getting familiar with how the plugins work at this point before I made any financial investment in a purchased one. On the free vst site download a compressor and get to work manipulating it to see what it does, and how it changes the sound. Then download another and experiment with it in the same fashion and notice that on the same settings the two compressors will most likely have different sonic characteristics. You don't need the most expensive equipment to make the best sounds, technique and experience will make even the shittiest vst plugin sound decent-- so start downloading and playing around. I'd recommend starting with compressors, moving onto EQs, then reverbs/delays, and then 'exotics' such as fx, multiband compressors (kinda like the result of an EQ and a compressor making sweet sweet love), etc. In all honesty, the fun part is done, you got to make some snazzy purchases and have a rig you can produce with, now's the work: you need to learn how to operate it and that's an art in itself.