While starting PC you need to press key to enter the BIOS. It can be F2, F9, F12 or delete. You can check which key to press on your BIOS screen while starting PC or on internet if you know brand of PC, name of BIOS etc. When in BIOS you need to find line that says boot priority. Here you need to change the first device to device you want to boot from. You can put here CD, USB, HDD or anything else. Set your device to first and save changes. Put CD, DVD, USB into right place and restart PC. After BIOS screen appears you should see screen saying booting from CD, DVD, USB. If the CD has a bootable files on it, it should boot you into whatever is on it. After that you will be prompted by OS on CD what to do. Select what you want and follow instructions on screen.