[musicman691]

While starting PT hold down the 'n' key on your computer keyboard and you'll be prompted to select your audio interface. You should be able to pick your iMac's built-in sound which really isn't all that spectacular given the super small speakers inside.

I know you're just learning the program and you'll get a good idea as to how things work but when you do decide to go PT all the way I suggest getting a regular interface and monitor speakers. You needn't spend big bucks to do so - a small MOTU interface and JBL LSR305 speakers will do you well and not cost much.