How can i erase all the stuff on my recordable cd-rw?

Both Realplayer and itunes will erase cd-rw.

Write over it or destroy the disk.

umm you cant but you can rewind to where you dont want it, then record over all that

You can use whatever program you used to write it that came bundled with your burner drive (Nero, Sonic, RecordNow, etc.) I am not sure whether the default windows CD burning application will erase CD's, but windows has a way of messing up CD's so I would strongly discourage you from even attempting to use it.