I'll second the the call for Audacity- it's free, it's fast, it's easy to learn and there are a lot of tutorials online. BUT, with Avid releasing a free version- keep your eye on ProTools. It's a cleaner (some would say more confusing) interface, with a lot more potential for fine control. I use Audacity for quick and dirty effects, but if I'm building a soundscape, I find ProTools a lot easier to work with.
Protools' interface is, I gather, similar to the also F/OSS Ardour, and yes, it takes some getting used to; they violate the traditional WIMP paradigms whenever necessary to make the things you *do* all the time easier to accomplish; it took me a while to realize it was a feature, rather than a bug...