Usually your computer (PC or Mac) boots from the operating system of the hard drive, but there may be times when you boot it from a bootable CD-ROM, for example when installing a new system or installing the running a program (cleaning a hard drive) that requires booting from an external media.
Steps
Method 1 of 2: Using a Windows PC

Step 1. Insert the CD-ROM into the computer
Place the CD-ROM on the player tray, decorated side up. The CD-ROM must be bootable, ie contain a Windows operating system.

Step 2. Open the Start menu (

).
Click the Windows logo in the lower left corner of the screen, or press the ⊞ Win key.
In Windows 8, place the mouse cursor in the upper right corner of the screen, then click the magnifying glass

Step 3. Click on / off (

).
The button is in the lower left corner of the start window.

Step 4. Click Restart
This possibility is offered to you at the bottom of the sub-menu that appeared.
- If you have any programs running, you will be notified. If you have nothing important to save, click Restart anyway.

Step 5. Enter BIOS
The key to press at startup varies from computer to computer. Often it is the delete or F2 keys. The phrase Press [key] to enter setup (Press [key] to enter BIOS) tells you which key to press.
Otherwise to know the key in question, consult the booklet of your computer or do a little research on the Internet

Step 6. Navigate to the Startup tab
To select this tab, use the arrow keys on the keyboard.
- On some computers, this will be the tab Boot options, which is exactly the same.

Step 7. Select CD-ROM Group
Use the arrow keys to highlight this device (white letters on a black background).

Step 8. Change the boot order
Press the + button until
CD-ROM group is in first place, at the top of the list in short.
If the + key didn't work, look in the bottom or right legend and see what to do

Step 9. Save the new settings
At the bottom of the screen you have indicated keys and among them there is one, F10 (Save and end), which allows you to save and exit the BIOS. Your computer then restarts on the operating system of the CD-ROM.
You may also have to validate the request by pressing the Enter key
Method 2 of 2: With a Mac

Step 1. Insert the CD-ROM into the computer
Slide the CD-ROM into the slot of the reader located on the edge (on the right). The CD-ROM is inserted with the decorated side facing up. Your Mac will only be able to start from this CD-ROM if it contains a minimum operating system.
Today, no Mac is sold with a CD-ROM drive. And so to be able to boot from such a medium, you will need an external drive that plugs into a USB port

Step 2. Click on the Apple menu (

).
It is on the far left of the general menu bar, at the top of the screen.

Step 3. Click Restart
This possibility is offered to you towards the bottom of the list.

Step 4. Confirm the restart
Click the Restart button, which will result in… restarting your Mac.

Step 5. Hold down ⌘ Cmd
The computer turns off briefly, you must immediately press and hold this key until the startup window appears.

Step 6. Then click on the CD-ROM icon
If it is an installation DVD of a certain version of Mac OS X, this icon will show this mention.

Step 7. Validate with the Enter key
Your computer will restart on the CD-ROM which has thus become priority and will remain so as long as you do not change the order.