Going to see a counselor about my mom, what to do/expect?

So my mom and I never get along and I do not know how to deal with her anymore so my dad thinks that I should go and talk to someone. My parents are divorced, by the way. My mom left my dad and she treats me and my sister bad as well. I am willing to try anything to get along with her better so I have agreed to try counseling. I am 19 years old and I have never been. What should I expect and do when I go there?