I would suggest using something you like, or combining two things. An example being, I love hot chocolate, and I also love wolves and what they do. So I can use those things for a username. CocoaWolf, MochaWolf, and so on. Or, you can think of things that sound good together. Absolutely love pineapples and video games? PineappleGamer - RetroPineapple. Or, if you want to go off the theme of making videos, you could throw some of that in. PaparazziWolf, PaparazziPaws, and so forth.
All in all, just have fun with it! You can always change your username later if you don't like the one you currently have. =)
I suggest have a username after something that you like. Or even make up a word or name that sounds cool to you!