Talk to a trusted adult or call the police.
What do you mean by "touches you"!? I think I just know what you mean, I know these kind of things so to prevent this first of all tell your best friend about it to take her or his advice then tell your mom about it but before telling your friend tell your mom and if she just doesn't do anything to prevent this then... Complain to the police about your step dad!
Tell your mom. If she doesn't believe you or doesn't do anything to make this touching stop, then tell your teacher after school. Your teacher will call a social worker who will talk to you about this. It will be very helpful for you to have dates and times the touching happened. Be clear about what you want. For example, the touching must stop. You don't want to be alone in the house with him. You want the social worker to see if he has done this to other kids before getting together with your mom. Please let me know what happens.