So i want to take the background of a video and put it in another video.The thing is i have people that are moving in the video that i want to take the background...I'm new in after effects so take it easy on me and sorry for my bad english.
See this page for an overview of compositing features to do the kind of work that you are describing:
Start here to learn After Effects:
After Effects is not magic. You can't remove people from a scene without replacing the empty pixels they leave behind.
Say you have a photograph of a little fountain in Rome. There are two people in front of the fountain. You take scissors and cut out the people. Now you need a photograph of the fountain without the people to fill the holes left by the people. It's the same in AE. there are advanced techniques for doing this trick but it's usually easier to go shoot the background again. The term applied to this layer is a "clean plate."