The best place for a newbie to start is the User Guide.
After Effects is NOT a video editing app. It is a layer-based visual effect, motion graphics, and compositing app that is incredibly powerful. You can organize and arrange layers any way you want in the Timeline Panel. You set the length of the composition in the Composition settings. Please take some time to go through at least the beginner tutorials so you have a basic understanding of the UI and the basic workflow.