Hello,
Don't worry, it's very easy. Once you've plugged your USB key in your computer, just reboot. At the very first screen when your computer manuacturer and model is shown, there will be a small message saying "press this key in order to boot on your key". You'll just need to press it, and "voilà".
Otherwise, you may need to see at a repair shop, because it's very likely that whether your ...