When you are happy with your mix, go to File - Export Audio, and pick a directory to save it in (hint: pick one that is easy to find or one that you will remember),and name the file. Select the bit depth you desire (16 bits if you are planning to burn to an audio CD), and choose Stereo file. Hit Export when you're ready, and it will create a stereo file of the mix. You can now burn this file in your favorite CD burning software.