Improve your feet and improve your passing. Youre job is to decide what to do with the ball once the 10 passes to you. Does it look like theres a chance of an overlap? Pass. Does it look like the defence is pushing up too far? Kick. Does there look like a chance to get territory and a line out? Kick. The best thing to do is just remember your role. Youre one of the brains of the team along with the flyhalf and scrummie. You'll need practice to read the game, but in essence you will be doing one of these three things. Kicking to touch, distributing the ball to your 13 or wing, or attacking the gainline by crashing into the defence. You can go to ground to slip an offload here. Which one of these options you do depends on how well you read the game, and how good you are at each of the three things. A Nonu type 12 is a fan of crashing into the gainline rather than the kicking aspect as Luke McAlistair would be. Nonu obviously does that as he's as bulky as some fowards and can usually slip out an offload to a running on player. McAlistair on the other hand would rather pass it wide or kick to the corners to get the team forward. I think I answered a similar centre question yesterday, but anyway a type I put there too is to watch the pro's. We've a much better advantage over past generations in that we can watch youtube video's all day when bored, try to watch some inside centres on it. See how the pro's do it. Model yourself on one of them who suits your physique (relatively to your age group ofcourse) and what skills your best at. The rugby world is rich in talents, you wont be short of video's. Good luck!

go to the gym and make yourself stronger. also, practice passing with a friend... as you get better at it, increase the distance