Being Nice to people who aren't nice to me?

People at my school talk about me and I cry. I continue to be nice to them and I don't know why. My mom told me that I am just a lilttle too nice and I have to put my foot down at some point in time. I tried doing what she said and they laughed at me. I talked to my sister and she told me to Fight to make them afraid of me. I am new to this school and I don't have any friends. That's all I wanted. My first day there I got beat up and I went home with bruises. Now I just want to go back to Trinidad and stay with my mom. What am I supposed to do? Help Me?