Most present-day desktop and notebook PCs have a key to select booting from any media type available in the PC. It is normally F11 key. When pressed, a boot device selection menu is displayed.
In previous BIOS versions, you may select boot device manually. Just enter BIOS by pressing Del or F2 key during booting, open Boot Settings (or sometimes simply Boot) section and change the boot sequence (CD-ROM should be set to the first position).