how to do the video commentary with you in the corner of the screen?
Since you didn't answer @Ryan's question, I'll assume your talking about video editing, not live streams. 

What you're talking about is a very basic editing technique called a video overlay. Obviously, you're going to need two video recordings for that, one of which is the screen capture. There are several pieces of software out there that will record your computer screen while you play your game (or whatever you're doing), e.g. fraps. The second video is the one that shows you. To record that, you will need either a camera or a webcam (careful with the latter, those usually provide poor quality). You record yourself, copy the video files to your computer (or connect the camera to your computer and have it save the recording directly to your hard drive) and import them to iMovie. Then you just have to put one video on top of the other and you're done – even though iMovie is a consumer-centered software with not that many features, it's sufficient to do video overlays. Here's a simple tutorial:


                
            

If you want to do that as a live effect while streaming, the required software and setup is a bit more complicated – there are several good tutorials out there.