The best way to learn is to join a Rugby club. There are coaches there that will assess your skills and physical build including speed and agility. Then they can help you gain skills for a position suited to you. You can also watch games and learn how and what the backs do - This for many is the first education they have in the sport.
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You will need the speed and experience to move out. Many good backs have been training or have played in the forwards if only in heavy training sessions - this improved the "big hit" effect when running a straight line at the opponent. Speak to your coach and discuss it
A back is one of the seven players that stands outside of the scrum in a rugby game and do most of the passing and running with the ball.