Right click the file, and one of the selections should be "Convert file to MP3". You might have to fiddle with a setting to switch between convert to MP3 and something else, but it should be fine.
Edit: I thought it was just WAVs that it converted.. oOo:
If you want to burn a CD in iTunes, I believe it will automatically convert AAC/M4A/whatever to WAV, but I don't know if you can just do it to a file without going through the burning?
Out of curiosity, why do you need to convert in that format Herr Tripe?
Thanks guys - Ya I need to do this because my PC's burner for some reason doesnt burn music CDs (Although data cd's seem to burn okay oOo: ) so I have to put the files on my home network and burn them on my families' system upstairs. It doesn't have itunes though, and doesn't read M4A files... (For some reason a bunch of my files are in this format - I didn't even realise)