If these are DRM free (so, depending on how recently you bought them from iTunes), you can convert them from within iTunes. In your preferences, under the “Import Settings” you can select the format of music you encode from a CD, which confusingly also controls what format you will convert files to. Set this to whatever you want your new file to be (MP3 in your case). Select your song(s) from your library. Under the Advanced menu there will be a “Create MP3 (or whatever format you selected) version” option. Do that.
The simple solution is to “upgrade” them to DRM free versions. Somewhere in the itunes store is a link that will do this. The cost is about 50c/song or $3/album. Once you have the DRM free versions you can convert as Schmidt explained above.
Yeah, sorry for the imprecision, it’s the m4p files that are a pain. For example, I’m trying to move some things to use on the PS3, plus using some for background music for some movies I’m editing.