Tips please

Hello,

So I engaged to this lovely gentleman and he has a 5 year old. I just want some good tips that can help my relationship with my step daughter. Sometimes it I hard because she doesn't listens to me and his family making seem like I am being too harsh on her. I do not have any kids and this is my first relationship with a man that has a child. I do not know anything about parenting but willing to do better. My mother raised me in tough love, she was always rough with me and I guess I was doing the same thing my mom was doing but I didn't realised. I love this little girl like she is my own but they are making it seem like I don't.