I open a door, and inside I want to put an image. When opening the door I want the image inside to reveal as I open the door. But I can't figure out the masking and layers in After Effects. Can't find any tutorials that'll fit on Youtube. I want the mask to follow the edge of the door, so the image inside slowly reveals. But I don't know how to put the mask. No matter how I do it, the mask turns out black. I started doing this in FCPX. The image inside is keyed(blue screen), so there's only a tiny person inside. I have now two clips. One where I open the door without anyone inside, and the same clip with the person inside.
Can anyone help me?
Sounds like you should start here: Getting started with After Effects
If you're not getting things like layers and masking, that's a great place to start. It'll cover a lot of things that you've probably not even realized that will make your AE life much better in addition to getting you the essential info you need to work like how to animate masks.
If you're determined to skip the foundational stuff (please don't skip it), go through these resources: After Effects Help | Managing and animating shape paths and masks