Be a good athlete. Develop athletic skills that translate well to professional football; Run track if you want to play speed positions, such as wide receiver and safety -- football players are measured for their times in the 40-yard and 100-yard dashes. Lift weights to develop good upper body strength and a low center of gravity if you want to play as a lineman.
Practice your skills as much as you can. Work out by yourself on individual skills, such as running pass plays or pass blocking. Play in pick-up games or with friends to work on team-specific skills such as defending passes and pass catching.
Play semi-professional or other professional football. Locate a semi-professional or professional team in your area and try out for the team. Play for them, if you make the team, for a couple of seasons before trying out for an NFL team; semi-professional teams are found in many regions of the country, while professional leagues include the Canadian Football League, Indoor Football League and United Football League (as of publication date).
Study as much as you can about football plays. Look for sources of plays and playbooks online or at a bookstore. Learn the plays specific to the position that you want to play, but learn how the plays being run by other position players work as well; often players try out for one position, but are asked to play another by the coaching staff.
Sign up for a tryout with a NFL team; tryouts are typically held twice a year by NFL teams. Compete and do well during the try out to get the attention of the coaching staff. Do well during the drills and the scrimmages they ask you to run. Show them you know how to play the game and can do well at it.