First off make sure you have all the effects and mutes on channels that you want. Then look at the top of the screen, there should be two little triangles called markers (these are used to tell the computer what part of the song to export). Slide the right one of these to the end of the song then do the following.
File->Export->Audio Mixdown the n a few options will come up. What you want to choose is Stereo Interleaved, 16 bit, 41000 hz(can't remember wether its Hz or bits a second or whatever-Its christmas and the beers are starting to take effect.Hehe) the wav or mp3 whichever one you want. Then finally click export or ok and that should be it.
Have a Merry Christmas and a happy new year
dave