I have a 2 yr old with my soon to be ex gf I'm moving out and want to bring the child with me so how do i become the custodial parent?

Assuming the mother is not consenting to this, you need to file a petition for custody in the county in which the child lives, and prove the child would be better off with you than with the mother.  The court's determination is based on what is in the best interests of the child, which is almost always decided after a hearing.  If it is not clear-cut that one parent is a better choice than the other, it could be a long battle, which might be better resolved with an agreement to share legal and physical custody.