If you are new you should start with the User Guide. There are tutorials on the basics like previews and rendering.
After Effects is NOT a video editing app. It is a motion graphics, visual effects, and compositing apps designed to create shots and occasionally short sequences that cannot be created in a non-linear editor like Premiere Pro. More than 90% of my comps are under 7 seconds and one shot. Most of my comps have somewhere between 10 and 30 layers. Most of the movies I work on are somewhere between 10 minutes and 2 hours long. Most of my films require visual effects to be used on about 20% to 30% of the shots.