As Son of Mixerman said, competetive software developers have leveled the playing field on the various software that can accomplish what you are after. Researching some of the names above wil help you decide what features are most important to you. Many of the software you wil find have a trial download so you can actually test it out before you buy it. There is even a lot of freeware out there that can help you get started.
Not knowing your level of experience in mixing/mastering, I would recommend spending more time learning how to use the equipment. Understand what a compressor/limiter does and what the settings are. Time-based processors (reverb, chorus, delay flange, etc.) what they do and how they are different from each other. No software alone is going to 'make you songs better', in fact, most of the time they will get worse if you don't figure out what you are doing before using them. If you already understand all this stuff, then tweak away and have a blast.
Best of luck.