The technique is called rotoscoping.
Type "roto" in the search help field at the top right corner of AE and check the community resources.
Check our this short video.
Thanks a lot ! Since you wrote the comment i am editing the angles like the video show. What have i to do now ? I am done with cutting ....
I am looking forward for your answer.
Once you have the mask working the way you want it to you use that mask as a track matte to either isolate What do you have masked or separate what you have my masked from the background.
If you don't know what track mattes are or how to set them up then type "track matte" in the search help field at the top right corner of AE and study up.
I don't know anyone that has successfully done anything with AE without spending some time studying. If a nonlinear editor like Premiere Pro was a car, AE would be an F-18. A couple of five minute lessons from your mom might get you out of the parking lot in a car but it's going to take a whole lot more to learn how to fly an F-18.