If you want to make an mp3 CD (to put a lot of songs on one CD, as opposed to a 'regular' CD), then you have to convert your AAC files to mp3 format first, then burn as mp3 CD.
To convert your AAC files, if they are
not protected you can simply change your import preferences to mp3 and then right-click on one or more songs and select "Convert to mp3...". The downside is that iTunes will create duplicates (the original AAC and mp3), and you'll lose some fidelity in transcoding like this.
If your AAC files are protected, then we're not allowed to help you get around the DRM on these boards.
{edited for TOU compliance}