There are a few ways to outline a shape in After Effects.
One way is to use the pen tool to draw a shape around the object you want to outline. Then, use the stroke effect to add a stroke around the shape. You can adjust the stroke width and color to get the look you want.
Another way is to use the track matte tool. First, create a solid layer that is the same color as the background. Then, place the layer with the object you want to outline on top of the solid layer. Then, use the track matte tool to select the solid layer as the matte for the layer with the object. This will create an outline around the object.
You can also use the rotobrush tool to create an outline. First, select the object you want to outline. Then, use the rotobrush tool to paint over the object. This will create an outline around the object.