Firstly I created a new composition and adjusted the appropriate settings. Then clicked layer new solid making it the same size as the composition.
Then I selected the pen tool and started to draw a closed shape. Effectively here I have cut out a mask. Using the selection tool I modified the shape. Then i twirled out the little triangle shown next to the layer and clicked on the drop down menu on the mask. To animate you click on the stopwatch then move the timeline along a little and then start to adjust the shape.
Adjust the feather and opacity to give the shape a nice finish. You can invert the layer and create as many paths as you want to make animation.
Then I looked at shapes rather than solids so I created a new composition and went on layer then new then shape layer. Under the shape tool theres many shapes you can play around with.
The last layer I looked at is the adjustment layer which is similar to the adjustment layer in photoshop. In After affects you can add adjustment layers on top of one another to add effects.