Live commentaries are kind of hard starting out, but eventually you will get the hang of it.
First thing is first, your microphone placement and input levels. The microphone should be placed facing your face (obviously) and away from your controller. It is important to have the input level of the microphone set so it can catch your voice, and not your controllers thumbsticks. I recomend recording without speaking for a game to get the microphone level correct.
When recording a live commentary, don't stress about what you are about to say. Just sort of let it flow. If you say something that you did not mean to, you can easily edit it out in post production. Relax. Think about the content that you watch and how they commentate. Maybe you like them because they are funny, or maybe it is because they take the game seriously. It is nice in both of these instances to have someone you can communicate with.
For thumbnails you can either make them yourself using paint or photoshop, or you can find someone on Freedom's GFX area that would be willing to do it for you (although some people do charge).
Hope that helps out man!