I am 12 years old and I want to know if I can decide which parent to live with.

You can choose where to live at age 18.

Before that, it is decided by the courts as to what is in your best interests. You may sometimes have a say in it, but ultimately, it is decided by a judge.

ANY legal question needs to have your location listed in order to get accurate answers, though.

As noted a child cannot make this decision. Only a court can. A court is required to make this determination on the basis of the best interests of the child. Depending on where you live as you get older a greater weight may be put on your preference. At 12 I doubt if much weight will be given.

If a change in custody is to happen, the non-custodial parent needs to go to court and request a modification of the custody order.