The best way to get great sounds is having great sounds to begin with. There's not much you can do with tracks that went into the computer sounding bad. I would start at the microphone level and make sure you getting the best possible sound for your liking into your computer in the first place. This means decent condenser or dynamic mics based upon your recording style and musical taste. All the plug-ins in the world can't make bad mics sound good.
Try using the search button on the BBS and look for what people think of good reverb, compression, and eq plug-ins. Many come in packages where they will bundle all these plug-ins, plus probably a few other things for a certain price. Some of them can get pricey, however, you get what you pay for. Ultrafunk make a good package along with TC and waves. You can get demo versions on most of their websites for you to try out and see which suites you the best.
Getting a sound that your happy with into the computer will most often give you that "professional" sound right there and with a little tweaking through plug-ins and mixing, you can get that sound you hear through commercial cds. It is definitely possible.
Reading alot especially on this website will help you a great deal in your journey or recording. Reading the manual alot helps too...
hope this helps,
Brandon