In some cases, repairing disk permissions are enough to cure crashes caused when launching Apple's Garage Band. When it's not, the easiest way to get back up and running is by trashing your Garage Band preferences. Go to your User Home page, then open the Library folder. Inside, you'll want to trash the Garage Band preferences inside the Cache folder, as well as the "com.apple.garageband.plist" and "com.apple.garageband.cs" files within your main Preferences folder.