congrats on your new mac :c)
in your Applications folder, and inside the Utilities folder in there, you'll find a program called "Disk Copy". You can open it up, and in one of the menus (the second from the left, I believe), you'll see an item like "Erase CD/DVD-RW". It'll ask you if you want to do a quick or full format - quick is usually fine, but full offers more compatibility, i.e. if you're going to rewrite it on another machine, like your pc.