Assuming you are using a Windows computer, to boot into the menu you would need to press the F12 key on your keyboard right after powering on your computer. This should bring up the Boot Menu for you to select your boot device.
If you are not presented with the Boot Menu, it is likely because your computer is not configured to do so. To check if this is the case, you can access your BIOS/UEFI (Basic Input/Output System or Unified Extensible Firmware Interface) settings. Depending on your motherboard manufacturer, this might be done by pressing a certain key during startup (usually DEL, F2 or ESC). Once in the BIOS/UEFI settings, look for an option called Boot Menu or Boot Options. If this option exists, enable it and save your changes before exiting. You should now be able to see the Boot Menu when starting up your computer.
Keep in mind that if you're trying to boot from a USB drive, you might need to change the boot order in BIOS/UEFI so that USB drives are checked before hard drives.