I like Wavosaur...
http://www.wavosaur.com/
It's easy to use, has great file support, and has a traditional, clear GUI.
It's important to know how to edit sound effects, doing some self-teaching first will help you a lot I think, because once you know how to chop up sounds, loop them, get rid of clicks, filtering etc - the more of that stuff you know, the easier it is to aquire and adapt sound effects for your projects. It's good fun actually, to take a sound and extract the useful parts, then have it play the part of a machine, or a gun, or enemy squeal, or anything in your game.
As for sourcing sound effects, well people are mentioning good sites for this already. I would say that I'd prefer to record my own sounds, but that's pretty time-consuming - maybe you have friends who wouldn't mind grinding some sounds for your game.
I have all the Soundmatter packs, which is a good resource of high quality sounds - there's not exactly bucketloads of sound effects, but what is there is great.
Health, Ammo, and bacon and eggs!
