I have played full-back too - and think it is the best position of the team.
You have to be good under the high-ball - make the try preventing tackle and join the attack when possible. If you play well then the whole team has a lot less to worry about: playing full back is a big responsibility.....
Defence positioning is difficult because it depends how you defend as a team. There is a very basic option. This where you stand deep, waiting for a possible kick. Because of this you have to learn how far your opponent kickers can get the ball back. Problems come when the opposing full-back joins the line and the ball is not kicked: then your teammates have to deal with an extra attacker. This is why defenders then drift to the outside: to catch extra attackers.
Another option is that you join the defensive line earlier to make up the numbers. Your blind-side wing has then the responsibility to take over your deep position.
Perhaps the best idea is to discuss this with your team and coach: you all have to be aware of these options.
Success and enjoy playing full-back!