You don't need to tell your mom anything. Just join when you get to college. Have fun.
ETA: no, parents aren't usually involved. Unless it's a "somone's mom who doesn't live to far away and always likes to make tons of food for occaisions anyway made us another ton of food for this occasion" type thing. Sometimes there parents weekends, but if you join a good sorority, you will have supportive sisters during days like that.
Just relax, take it as it comes, and have a blast. Dues aren't the same everywhere. Remember: it is YOUR college experience. Not your mom's.