If the songs are in iTunes already, make a new playlist and copy the songs you want on CD to that playlist (just drag them to the playlist's name on the left pane). Then put in a blank CD and (assuming in edit->preferences you've set it to burn audio CDs and not data or mp3 CDs), click the 'burn' button and it'll make the CD for you.
If the files aren't already in iTunes but are on your iPod, you can use software such as yamipod to get the songs from iPod and onto your computer to put into iTunes:
http://www.yamipod.com