Instructions
Load the game to the main menu.
Select "Multiplayer". Depending on the game you are practicing, the exact wording and steps vary.
Select the split screen option and start the game.
Practice aiming at your opponent. If you are new to games in general, you can start off by controlling both players yourself, letting yourself aim at a still target.
Ask a friend to control your opponent. This allows you to practice at shooting a moving target. To ease the progression of your practice, have your friend only move around and not try to attack you.
Have your friend return fire. Since your online opponents won't just be harmless paper targets, it's best to practice trying to neutralize an opponent who is trying to do the same to you.
Play the campaign. All first-person shooters have an offline campaign you can play. This gives you about five or six hours of practice, and you can take your time defeating your opponents as well.