Adobe provides plenty of help resources to assist you with learning After Effects. When you open After Effects, you’re offered a range of help including hands-on tutorials:

You can also add a ‘learn’ panel to the main interface. Select ‘Learn’ from the Window menu:

To view Adobe’s online tutorials, select ‘After Effects Online Tutorials’ from the Help menu:

To create a new composition based on the resolution and framerate of a specific video file, you can click ‘New Composition from footage’ instead of ‘New Composition’. This saves having to set the resolution and framerate manually when you know your output is going to match the resolution and frame-rate of a specific imported clip:

Compositions Overview from Adobe (click to view)