Wheres the best places for Sports/NFL betting online?

I'm curious where the best places for sports betting is online? Mainly for NFL games. Ex) Patriots are 3 point favorites over Broncos. Where can i place a bet on either team? Whats the best site/s for that? and how exactly does that work? i understand that if i put money on team A to win and team A loses, i lose the money. But what if my team wins? How do i get the money ive won? Thanks for your help. Also any other other sports/NFL betting advice/tips etc are appreciated. Thank you.