I'd recommend just going to training and letting the coach try you in different positions.
When i started at rugby at 14, i was a very quick sprinter, and as a result i was on the wings, or at centre. I wasn't really smart enough tactically to play a key position like fullback, flyhalf or scrumhalf.. i didn't know the rules enough then.
Then, as i got a year or two into it and was becoming a regular scorer, it was very obvious that although i was not a flanker or forward if i had an opportunity i liked to join rucks as a back when it looked like we was going to lose the ball. I wanted to be in the action, as really did not like to lose the ball while i stood there, confused.
They tried me at blindside flanker and it was perfect for me. I was in all mixing-up, i had the determination to carry on after getting stamped, raked and allsorts, as it just made fueled me even more. I just seemed to have a forward attitude.
I was not that big or heavy to be a typical flanker, but i was very quick off the sprint with a standing start and i was a reliable tackler, i'd punch above my weight and height. I also led like a captain, i used to shout my lungs out to gee us up.
Unfortunately or fortunately, i can't decide.. the 1st team needed a centre and i was playing centre in the last couple of games for the 2nds.. so they tried me and i was a hit there as well, but as a back you have to be far more tactically astute and our coach at u-16's hadn't really even started making setpieces for the 2nd's, so i had a hard time even figuring out a basic play.
So, unless you have the pace and agility of a cheetah, then i'd recommend forwards first of all.
Here is what i wished i was taught when i played in hindsight:
As a forward.. GET A fitted MOUTHGUARD from a dentist. You won't have any teeth if things get tough.
1. Make training as tough and hard as possible. Tackle until you can't tackle anymore, and then some. Get tough or **** off. Fit in, or **** off. Run and crash ball the living daylights out of anything that resembles a human. Get used to getting mauled in rucks and mauls, you will get bitten, kicked and anything else a player can do to get the edge over you. Trash talking is fine, as long as the ref does not see it.
2. Make sure if you bulk up, it is muscle and try to eat better food specifically. Try to improve on things like agility if you are lacking. Any player on any position needs agility.
3. Learn some other skills, like passing, kicking and general tactics like a hand-off and a step. It doesn't matter if you are not 6'3'', if you are quick off the mark and have a step you can beat players bigger than you and go pace to pace with the fastest backs. You never quite know when you will intercept a ball and need to use a step or grubber kick when on 1 on 1.
As a back..
1. Make training as quick and energetic as possible. Run until you can't run anymore. Make sure you know your positions for defence and offence. Buy a book from a shop specifically about the rules/tactics of the game, as you will need to know about the mark, and what happens rules wise around a goal area on defence. Learn how to drop-kick and clearance kick.. you will never know when you are down sh*t creek and need to something different, like a chip kick. Get your feet comfortable kicking a rugby ball to different weights, like as far as you can and also with some accuracy.. like kick it along the ground for exactly 10m etc.
Never try to do anything that is not safe or pre-planned on your debut. You will get slaughtered. Don't run when you can save/kill the ball.
2. Make sure if you bulk up, it is muscle and try to eat better food specifically. Try to improve sprinting, fitness, agility and stamina. You want to get to the stage where if you can score 15 tries by yourself in 20 attempts, you can't and would then go some more, or at least want to. Get the better of your opposite number time and time again until you make him cry. Do whatver it takes that is legal.
3. Learn some other skills, like rucking and getting into a ruck. Often, a player with pace will find himself in a position which is isolated and you can only do one thing.. dip the shoulder and charge at them, but make sure you turn 180 towards your goal line, so they will have to ruck over you.
It doesn't matter if you are not 6'3'', if you are quick off the mark and have a step you can beat players bigger than you and go pace to pace with the fastest backs. You never quite know when you will intercept a ball and need to use a step or grubber kick when on 1 on 1.
Other than that, watch a real game of rugby as try to get a perspective of the rules as a viewer and as a player. This is useful because making an infringement in a ruck looks different to if you was standing and watching it. You will learn not to make infringements quickly.
Oh.. and enjoy yourself. The game is supposed to be challenging, but also fun too. Express yourself in whatever way you feel, you will make a mark in a team if you use your voice and lead by example.