This forum is an absolute gold mine. Here's another question.

[LunaSea]

9. It's different alright

It's free.
Audacity runs on either platform, as does Garageband. Audacity has a simpler interface, Garagebands' is kinda goofy at first but once you've spent a bit of time with it, it'll make sense. GB has an awesome playalong drummer feature in its most recent version which actually sounds quite good, not that you are likely to need such a thing.
GB can be a bit sluggish to startup, but it's also chock full of instruments you can play as well as amplifier modeling and an array of effects pedals, but you'd really want a midi keyboard for that (starting around $35.00).
There are many others not listed at Reverb.com, these were just suggestions from site visitors.

I'd recommend trying Audacity first, it's quite straightforward and simple (and free!) and has quite a few features.
Probably the best place to start. You can always move to other DAWs later.