if your awkward or nervous just do it anyway, practice makes it easier and easier every time, first time though, pick a game you know and love and whenever you get stuck for something to say just talk about what is happening in the game at that exact point, what your thoughts on it are and what you plan to do next. On occasion as well if you're very stuck for words just to give yourself time to think up something else to talk about you can summarize what has happened so far and such. What you talk about isn't as important as keeping a positive attitude and actually enjoying the game, an audience can tell when a commentator is having fun and they will enjoy the video much more.