How do I man up at football?

Im 5'7 and 184 lbs, so im the perfect frame for Rugby League. I joined my school team yesterday but i couldnt tackle, im so scared that ill end up breaking my neck or something. I also almost always end up running at an angle to avoid being tackled. And this was only training! Next week were versing huge islanders that are 10 times as big as the people on my team. What do i do?