How do I replace background? in After Effects

Just create a mask using the Pen tool (g), and then press Alt/Option + m to set the first keyframe, move down the timeline until the action pauses or changes direction, and adjust the mask. Repeat looking for arcs in the motion, then fix any places where the matte does not follow the motion quickly. 

Here's a tutorial that shows basic Roto using a solid layer with a blend mode so you can easily see through it and a motion-stabilized shot of a dog. It might get you started.

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If there is a lot of camera motion in your shot, you can also use Mocha AE to track the motion and create the roto masks. 

That should get you started.

If there is no camera movement, you can animate a matte that makes a hole in the footage, then distort your replacement layer with Corner Pin or make it a 3D layer and move it into position. It's called Rotoscoping.

If there is camera movement, you'll need to do some tracking, apply that data to your Rotoscope, and apply it to the background plate (layer).

If we knew a little more about your shot, we could give you a more concise workflow.