How do you make boundaries?

Hi, I'm in 8th grade, and my inability to set boundaries has been a lifelong problem. My friends insult me, throw me on the floor, and I don’t do anything about it.

Today, on my way back home with my friends, one of the larger ones randomly picked me up by my shirt and started ragdolling me. I can't keep dealing with things like this anymore. I'm constantly on alert around everyone, and the paranoia is messing with my daily life.

Could anyone help me figure out how to set boundaries? I haven’t given much context about my situation, so if you have any questions, feel free to ask.

Thank you in advance!