The basics of After effects...
Step 1: Setting the Right Settings ;D
First click on composition and fill out the pixels your camera uses 1280x720 being regular hd and 1920x1080 being full hd anything below hd that most cameras uses are 480, 360, 240...a toaster
Step 2: Duration
Now once we've figured out how to set the pixels you now have to choose the duration of the video down below you'll a section in yellow that says duration highlight the numbers and input how long you want your video to be. Finally click ok.
Step 3: Insert Videos
Now click on the File button and scroll down to import it will open a window where you can scroll through your files choose the ones you want and then upload them to your workspace.
Step 4: Editing
Now drag and drop your video into the timeline
Step 5: Whats the Timeline and How Do I Use It?
The timeline is where your video will appear by seeing it here you'll be able to shorten cut lengthen or layer videos