Set up iTunes to your preferences in the preferences panel. iTunes is the best audio player for Mac hands down, although there are several iTunes controllers,
bytecontroller is the best free one IMO and does just about everything the shareware ones do.
AAC is a better format than mp3, smaller file size better quality, but I see no prob with my 192+ kbps mp3s that I cop online. However I do encode all CD rips to AAC.
And while we're at it SRS iWow audio plugin is a must for Mac audiophiles.
DL it here
http://www.megaupload.com/?d=PDOHNIAR. It greatly enhances your audio experience, you'll not want to listen to music without it.