I have a clip of a house filmed from the outside, where we can see a window. I'm trying to add some light to make it look as is there was a light inside the room of the window when we filmed it. Is this possible? Can anybody tell me how to do it?
I don't really know of a tutorial on this exact kind of thing, but if you understand how to use transfer modes, solids, masks, and keyframes, you should be able to pull this off pretty easily. Those are largely the basics of after effects.
I would check out those tutorials. Start with the "Getting Started with After Effects CS6" part, which isn't at the top for some reason. Others can probably recommend other places to learn. After Effects is a massive program with massive potential, but the learning curve is just as big. Take your time, watch as many tutorials as you can, and practice, until you understand how to do most of the basic stuff, it's really hard to explain things without a lot of typing.
For some more advanced things, check out Video Copilot, it's where I learned almost everything I know about AE.
If anyone else has tutorials, feel free to give more.