I assume that you have downloaded the .iso image for backtrack?
Simply burn the image onto a CD using a suitable software such as the excellent K3B (or Nero if you have a Windows machine).
http://www.k3b.org/
Just click on the .iso file that you downloaded and K3B will do the rest.
Put the CD in your CD/DVD dribe and reboot. As long as your BIOS is set to boot from CD you will be up and running. (Usually press F2 or similar at boot to go into your BIOS to change it if necessary.)