Some actions require that you boot (boot means "start") your computer from a CD. For example, when your Windows does not start, or when you need to check if your computer random access memory (RAM) is fine.
Most modern computers are set to boot from CD or DVD first and if there is no bootable media in CD or DVD drive then from the first hard disk.
To boot from a CD, you must insert the CD and then restart your computer.
For changing boot order, you have to use a system setup utility, most often called BIOS (Basic Input/Output System) setup or CMOS (Complementary Metal-Oxide-Semiconductor) setup. Newer PC-s might include a BIOS replacement called EFI or UEFI. You can access the setup utility by pressing one or more keys on your keyboard before your operating system (Windows or Linux) starts. As there are very many different computer manufacturers, these keys or combinations are different, but they are almost always displayed on screen. These shortcut keys are also in the manual that came with your computer. Most usual keys are F1, F2, F10, F12 and Del (also Delete). On some computers you might have to press Esc key first to see options list.
On some computers you can select your boot device by pressing F8 key, but most common ones are F9 on HP, F10 on Sony, F12 on Lenovo, Dell and Acer, and Esc on ASUS.
On some computers there are also keys for quickly selecting boot device. This means that you do not have to use BIOS setup if you do not want to change boot order permanently.
Please do not change any other settings than boot order! Any BIOS/UEFI setup utility is a very powerful program and you can make your computer inoperable when you mess around with it!
Example one
As you can see from the picture, there are shortcut keys at the bottom of the screen. You can press F12 key to quickly select boot device this time. Then select CD-ROM with up and down arrow keys and press Enter to confirm (make sure the bootable CD is in the CD/DVD drive before this!).
To change boot order permanently, you will have to press Del key on your keyboard to access BIOS setup:
Use down arrow key to move down to Advanced BIOS Features, then press Enter key once:
As you can see, First Boot Device is 'CDROM', Second Boot Device is 'Floppy' and Third Boot Device is 'Hard Disk'. Always read the instructions at the bottom or the right part of screen to see how to change and select values. In this example you can use +, -, PgUp and PgDn keys (PgUp and PgDn are short for Page Up and Page Down keys).
As also described at the bottom part of the screen, you will have to press F10 key to save changes. You will then be asked if you want to save changes and exit (this means restart), press Y key and then press Enter. Make sure your bootable CD is in the drive now!
Example two
This computer displays a full-screen logo and again shortcut keys at the bottom of the screen. The fastest way here is to press Esc key, choose CD-ROM from the menu with arrow keys and then press Enter to confirm (make sure your bootable CD is in the drive before this!).
If you want to change boot order permanently, press F2 key while this logo is displayed:
Some letters are red in the top menu, this means that you can open the menu by holding down Alt key and pressing the red letter key - for example, you can open Boot menu by pressing Alt+B.
Open Boot menu, use arrow down key to select Boot Sequence and then press Enter key:
And again, the instructions are at the bottom of the screen. Press Tab key twice to activate device selection. The instructions will change to 'Press PageUp and PageDown to switch the device sequence'.
Move to CD-ROM with arrow up and down keys, then use PageUp key to move it to the first position in menu. Press Enter to close Boot menu.
Open Exit menu by pressing Alt+X on your keyboard, choose Save and Exit with down arrow key and press Enter twice to save new configuration and restart your computer.
I hope these two examples gave you enough information to change boot order on your computer. 
