I am legitimately jealous of the blank slate you have right now. You have the potential to put on some serious and quality gains if you do it right and stay consistent.
That being said, you are small as hell right now. So especially with a sport like rugby in might, size and muscle mass should be your number one goal.
I think the first and most important lesson when start the iron game is that lifting and strength training will be a marathon not a sprint. It will take a long time, possibly years to get where you want to get. It can be a super fun process along the way! Always remember that progress is always progress. If in a month or two you can bench 55 instead of 50, you should be proud. I wish i had understood that when i first started!!
The second thing i think is important is that you pick a quality program, and you stick to it for the first 6 months. I would honestly suggest Jim Wendler's 5/3/1 program, the Big But Boring template. Maybe even the "For Athletes" version
The 3rd and probably the most crucial thing i want to stress is nutrition. You are a small ass kid. Nothing wrong with that but you need to realize that right now, so you can work above it. As to nutrition, in the beginning, you shouldn't worry about macros or calorie percentages, and ESPECIALLY not worry about supplements. Maybe in a few months get a good whey protein, but for now, eat 3-4 meals a week, every single day. Do not skip a meal every. Meat/protein/eggs, carb like rice or pasta, and lots of veggies. Fill up on those., every single day. Again do not skip meals, even on weekends. Get second and third helpings if you get the chance.
Again YOU DO NOT NEED SUPPLEMENTS right now. Those will come later when you understand what they do and how to better use them. You have money for supps now? Go spend it on the "5/3/1" book and read and reread it. Then take thats left over and buy some chicken breasts. You will be better off.
Most importantly, enjoy being involved with a process that lets you literally improve yourself physically and mentally every single day. And ask questions when you need it. Good luck man.