GarageBand is great--it does 90% of what pro-level software does, and it does it in an amazingly simple way
The simplest way to do it is to get a 1/8 to 1/4 adapter, and then using the adapter plug your bass into the audio input of your mac.
Probably you'll get 2 problems with that method--a very low input signal, and "latency" or a delay between when you hit the notes and when you hear them. But maybe not
Anyway, once your bass is plugged in, go to preferences, choose audio/midi, and set it to "built in input." Then choose "new track" from the track menu and click "real instrument. Then you can choose "bass" and if you want, choose from a bunch of preset models
Here's an example of a simple GarageBand project--a friend asked me to do some music for a podcast. It's all loops except the bass track, which is me on a diingwall five
http://chnm.gmu.edu/courses/omalley/podcast.mov