First thing to do is access the BIOS. Its a case of looking at the screen when the machine is booting up and making a note of anything that says "Hit del or F1 or enter or esc or whatever to enter setup" If it doesn't say anything press the Del key and the ESC and F1 at the same time whilst the machine is booting (at the beginning when you switch on)
Once in the BIOS you need to look for "boot order or sequence ETC: It might be under advanced options.
You will not have a mouse so you will have to use the up and down keys. Use the keys until you highlight CD and either press enter (it depends on your system) or use the + key whist holding the shift key to send CD to the top of the list.
Save and exit.
Restart and follow my above advice, you will be asked to "hit and key to boot from Cd-ROM"...hit any key.