By default, your computer starts up by reading the operating system information from its built-in hard drive. You can change this by telling it to prioritize plugged-in USB items over the built-in hard drive.  The settings for your computers boot behavior are in a menu called the BIOS, which is a pre-startup menu you can access by pressing a key specific to your computer model as your computer turns on. In order to boot from a USB flash drive, the flash drive must be set up as a bootable item, and it must have a disk image (ISO) file with an operating system or similar service installed on it. The key that you press to access the BIOS varies depending on your computer manufacturer and model. You can look up your computer's BIOS key by typing your computer's manufacturer and model name, along with "bios key", into a search engine and reviewing the results, or you can consult your computer's manual if you still have it. Most computers use one of the function keys (e.g., F12) as the BIOS key, though some computers will use the Esc key or the Del key. The USB flash drive should plug into one of the rectangular USB ports on your computer's housing. On a laptop, the USB ports are typically on the sides of the housing, while desktop computers typically have the USB ports on the front or back of the tower. . If your flash drive isn't already bootable, use the Command Prompt or one of the Windows installation tools to make it so. Copy the ISO file you want to boot from by clicking it once and then pressing Ctrl+C, then open your flash drive and press Ctrl+V to paste in the file.  For example, if you're attempting to install or use Ubuntu Linux from the flash drive, you would paste the Ubuntu ISO file into the flash drive's window. Skip this step if you created a bootable USB using the Windows 7 or 10 installation tool. Before you proceed with accessing the BIOS, you'll want to save any open work and then close any open programs to avoid losing your progress.

Summary:
Understand how booting from a USB works. Determine your computer's BIOS key. Plug your USB flash drive into your computer. Make the USB flash drive bootable Add the file to which you want to boot to the flash drive. Save and close any open work.