I want to change what people think about me at school, please read details. Thanks!?

So at school I am known as being nice but weak. This kid at school. He has been such a jerk to me. But its not about that. I wanna be known as nice, yet strong. I wanna be known as the guy who you dont wanna pick on or bully. So what are any exercises with and without dumbells that I can use to get strong. What are good exercises without equipment that I can do that will help me to get buff/strong. Please dont say take kickboxing/karate. Because I have $0.00, because I'm 15. Thanks so much. :)