As simply as I can put it, you start your PC and strike a key to interrupt the boot process and enter a "setup" screen. The key you need to strike varies with each computer. Maybe if you can tell us the brand and model of the PC, someone will know which keystroke to use but it's often the Esc, Del, F1 or F2. It might be that the key to use flashes briefly when you first start your PC and you've just never noticed. Once in the setup screens, you look for a one that refers to boot order. Your desired boot device needs to be moved to the top of the listing. Often, once you're in the correct screen, instruction on how to do this is off to the right side of the current list.
Again, PCs vary and there is no way here for anyone to offer step by step. This is really something not that hard to figure out and I'd recommend you give it a try. The best learning comes from what be bang our heads on until we succeed and not what someone else guides us to do. Good luck.