Know the basic rules of football. You won't need to know every specific rule of the youth football league before you become a referee in that league. You should know, however, how points are accumulated in a football game before you attempt to become a referee in a youth football league.
Make sure that you're in good enough cardiovascular shape to be a youth league football referee. A youth football league probably won't play games on a 100-yard field but you'll still be on your feet and running for a long period of time. Some youth leagues even require referees to work more than one game on a weekend.
Contact the youth football league about your desire to be a referee. You will have to set up a meeting with one of the league organizers before you can even begin the necessary training.
Fill out the required form for a background check. This is standard when doing getting involved with any youth sports league. You may or not be fingerprinted as well.
Attend at least one day of youth league football. Most leagues will actually require this so that you can get a feel for the rules and tempo of the league. This also gives you the opportunity to talk with other officials in the league and get some advice about being a referee in that league.
Read over the official rule book of the youth football league. Different youth football leagues will have different policies regarding penalties, substitutions and change in possessions.
Pass any necessary exams. Some football leagues will ask that you complete a multiple-choice test. This test will most likely contain no more than 50 questions and all answers will be located in the rule book. You will need to speak with league officials to know exactly how many questions you must answer correctly to pass the test.