iTunes can convert wav to mp3 or AAC. I'd chose AAC as it's got about twice the compression rate as mp3. AACs will run better on your Mac.

Garageband should have come free with your computer, it's part of the iLife package which has been standard software on every Mac for years. Spotlight it, it should be in your Applications folder.

Audacity is a pretty nice audio editor / mixer. I like it better than Garageband.