Are you currently in a relationship with the other parent? If so, work it out with the other parent, or if you can't, go to couples/marriage counseling.
If you are not currently in a relationship with the other parent, is there a custody agreement? If so, what does the custody agreement say?
If you are not currently in a relationship with the other parent, and there is no custody agreement, then either work it out with the other parent or get a custody agreement.