If they're in iTunes, you can use the burning system built into iTunes.
Just make a new playlist, drag the songs you want into it, and put a blank disc in. The 'burn CD' button (I think) will show, and then you can choose how to burn the CD --- choose audio CD. (On older iTunes versions this setting was in edit->preferences.)
You can burn any iTunes-supported audio files to CD, including mp3, aac (also known as mp4 or m4a), and protection aac files that you may have bought from the iTunes store.