There are a few seconds in my video in which (unnecessary) people come into frame. I am trying to paint over it but I am just making a mess and getting keyframes and all confused. The part is in the middle of the video. When I paint over the spot, the effect layer just goes on till the end! Help! Really Urgent. I am a beginnerish guy.
Nobody can tell you much without actually seeing the shot. Painting is usually the poorest way to fix such issues, anyway.
If you are using clone stamp or paint then I would suggest that you type clone stamp or paint in the Search Help field at the top right corner of AE and study up on how to use that tool. You've probably set your paint to duration to constant but you won't know what that means unless you study up on the tool you are using. AE is a very complex app and there is very little that is automatic. You have to learn how to use the tools and the interface.