This is very difficult to do convincingly without a great deal of pre-production planning. Basically you have to learn about masking, tracking if the camera or the subject is moving, and layering. If you are new to AE it's going to take you a while just to figure out the basics. if you just want to add someone to a frame then cut them out of another video using masking and position them as a new layer in the existing video.
Research rotoscope to see how masking moving things works.
I know this is a huge thing but would you be able to do it for me. Please!
Depending on the shot this could take the better part of a day. Depending on how much you want the new person is actually in the shot this could take the better part of a week. Without some very specific details about what you want and being able to see the source footage I don't think you're going to find any volunteers that would do something like this for free... I'd be happy to take a look at your footage and give you a bid if the footage and the concept meet my standards. That's what I do for a living.
how to remove and insert person in video. I have a birthday celebration video. i have a pic of someone. i want to replace the birthday boy from video the person from pic. In video i want to keep everything else like music, other people, decoration, movements, etc. I just want to replace birthday boy with someone else from pic. Let me know if anyone knows how to do it. Send me the directions for it. Thanks.
It would be simpler to just place the new picture over the old person if possible. You could try to paint the old person out (though may not be very simple using AE). And use pen tool to create a mask around the new person. Though if the new person is just a single picture, it's probably going to be obvious that they're still while everyone else is moving (unless you tried to make it look like the new person is moving). Though replacing the original person with moving video of the new person (say against greenscreen? or just rotoscoped) might give better/more realistic result.
So roughly: load original video, create a composition with the same settings as it, put the original video on a layer. Load the new picture of the new person. Put the new person on a layer and draw around that person with the pen tool. Position the new person in the composition, scaling if necessary. Try to position the new person/other things so the old person isn't visible or paint the old out. If the camera is moving you could reposition the new person layer (using the tracking tool might help - or stabilise the shot). Render the result.
can you give me your email?