setup your comp with a transparent background, put your source picture on it and just use the eraser tool to takeout what you dont need. Drop in the desired background below it on the timeline.
Answering a question like this is like explaining any complex process. Online turorials will be a lot better than anyone can be within a forum. How do you make wine or cheese or what does each part of an engine do?
I can't personally vouch for this site (or pop-up risk, et-cetera), but he has several tips in the Layers section that might immediately help you with your question.