If I want to move with my mom but my dad has custody of me, what do I do?

Your mother have to go to court and apply for a "modification" of the custody order. The court will then look at what is in your "best interests". The fact is that you cannot decide with which parent you want to live. However, based on your age, your preference will be given more consideration by the judge (typically, the older the child the greater the weight is given to their preference).