Now that you have a Tails device you can shutdown your computer and start using Tails without altering your existing operating system.
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.
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.
Read our specific reporting guidelines when Tails does not start.