Most computers do not start automatically on the TailsUSB stick, but you can press a Boot Menu key to display a list of possible devices to start from.
[[!img install/inc/screenshots/grub.png class="screenshot" alt="Black screen ('GNU GRUB') with Tails logo and 3 options: 'Tails' and 'Tails (Troubleshooting Mode)' and 'Tails (External Hard Disk)'."]]