If you want one DAW for everything then nowadays Logic is definitely the one to go for. Fantastic program in its most recent form (Logic Pro 8 ). It comes with a selection of good (and sometimes very good) plugins and virtual instruments and is generally very easy to use. There's also a HUGE library of awesome samples which you may want to think about before you install them (~30GB worth).
For the money Logic is a ridiculous steal and the Logic Studio box comes with Logic Pro 8, Mainstage, Soundtrack Pro 2 and all the aforementioned samples, plugins and instruments. A couple of years ago all of those programs together would have cost you well over £1300! I think Apple are offering it so cheaply now because it's SO good, and SO little money that people might just go out and buy Macs purely for Logic. If you can get Higher Educational discount like I did then it's only £118 if i remember correctly. Probably my best buy of all time.
Protools is also very good, especially for tracking; it's a lot quicker to use than Logic, and has a very vast multitude of key commands - it's extremely intuitive and once you've worked it out then it's going to be the smallest barrier between you and your artistic vision I'd imagine. That said; I'm boycotting it due to the company that make it (Digidesign) being bar-stewards; to use protools you HAVE to use one of their audio interfaces which is ridiculous.
For simpler work Garageband is great - perfectly adequate for most people - I know a lot of bands who have recorded, mixed and mastered whole albums with only garageband. It's basically just a cut down version of Logic.
Welcome to OSX :)
