You need to run a screen recorder application then do some recording settings. (things like system sound settings, mic settings, etc.) There’s a free web-based application can help you to finish this task. You can use it to add some annotations on your gameplay video when recording. It also allows you to upload video to YouTube once recording is done. You will get more info about it from this guide.
"How many times have you wondered how in the world people put their gameplay footage from Xbox 360 and PS3 on YouTube and have such good quality? One YouTube director is creating a series that is a walkthrough for getting started."