No secret around here, I am a Pro Tools guy. I use it in my project studio. I have also used Abbelton Live, Reaper, Garage Band and Tracktion. I have Audacity loaded on a PC, but don't really know much about it. I can get it to work, but I don't really use it.
However, since you have Garage Band, that is where I'd start. See how you get along with it. Garage Band can also take advantage of plugins or VIs also that you may wish to buy. Lots of things you can do with it.
Pro Tools First would be a GREAT option if you had any idea at all about moving into Pro Tools. It is free, but it does have limitations, such as no mp3 export.
There was a time when Pro Tools would not export or bounce mp3 without an add on converter, and kestrou is correct, you can find all kind of converters for free that will convert broadcast wave to mp3.
Good luck, and have fun.