When you start working with After Effects, it will probably be overwhelming. There are tons of video tutorials online, but most of them focus on a particular effect and don't offer structured training that help beginners to get started.
If you are new to After Effects and don't know where to start, I recommend the following free training series
In this guru lesson you learn how to use masks and adjustment layers. We give the guru a dark face and add a vignette to the clip to make it look more interesting. We also cover the per point mask feather available in After Effects CS6 and newer versions.