Now that you have a Tails device, you can shutdown your computer and start using Tails without altering your existing operating system.

If you are using a DVD

Put the Tails DVD into the CD/DVD drive and restart the computer. You should see a welcome screen prompting you to choose your language.

If you don't get this menu, you can consult the Ubuntu documentation about booting from the CD for more information, especially the part on the BIOS settings.

If you are using a USB stick or SD card

According to your type of computer, read the instructions for either PC or Mac.

PC

Shutdown the computer, plug your device, and start the computer. You should see a welcome screen prompting you to choose your language.

If your computer does not automatically do so, you might need to edit the BIOS settings. Restart your computer, and watch for a message telling you which key to press to enter the BIOS setup. It will usually be one of F1, F2, Del, Esc or F10. Press this key while your computer is booting to edit your BIOS settings. You need to edit the Boot Order. Depending on your computer you should see an entry for removable drive or USB media. Move this to the top of the list to force the computer to attempt to start from your device before starting from the internal hard disk. Save your changes and continue.

For more detailed instruction on how to boot from USB you can read About.com: How To Boot your Computer from a Bootable USB Device.

If you have problems accessing the BIOS, try to read pendrivelinux.com: How to Access BIOS.

Mac

Shutdown the computer, plug your device, start the computer, and immediately press-and-hold Option until a boot menu appears. In that boot menu, choose the entry that reads Boot EFI and looks like a USB stick.

Then, Tails should start and you should see a welcome screen prompting you to choose your language.

If this does not work, then you might need to install rEFInd.

Installing rEFInd replaces your original bootloader.
This could cause your Mac to not boot. It is recommended to create a full backup and know how to restore. See Apple's instructions.

Problems starting Tails?

Read our specific reporting guidelines when Tails does not start.

Using a virtualization software

See the corresponding documentation.