I really want to play Football, but everyone keeps bringing me down... Help...?

Ok I really want to play Football. I'm going to be a Freshman in High School and I signed up for Football. Everyone is looking down on me saying that I'll get injured and everyone will break me. I understand that, It's part of the game, i'm willing to take hits for the team...I love the game and i'm serious about this, but like i said, everyone is looking down on me saying i can't do it without even knowing what i'm capable of.... I want to be running-back. I was always a fast runner. I ran the mile at my school in 6:14. I am 5 11' and 145 pounds. On Monday we will be starting 4 weeks of weight training at the school. I'm going to work my butt of before tryouts in August. Look,i understand that i don't have a lot of muscle on me and i was always considered a loser at school, but i have fast legs and a whole lot of confidence! I'm willing to work my hardest and not back down! Should i try out? Is everyone else right...? Please help.... I'm sorry for all the writing but this is important to me.