If I am 15 can I choose where I want to live?

You don't have an absolute right to choose. However, in determining custody, where you'll live, visitation rights, and all the related things that will affect you, the family court is supposed to look after your best interests. And while they do not have to obey what you consider to be in your best interests, the court should take your wishes into account. The best thing you can do is, whenever speaking on the record, to the lawyers, to the judge, or to some other court functionary or person appointed by the court, is to tell them what you want and why. Speak simply, plainly, and honestly of what you think will be best for you and why; provide as much concrete details or reasons as possible. What you say is supposed to be heard, and it should be among the factors that the court weighs in determing where you will live.

First of all, the parent seeking custody (i.e. the parent with whom the child wants to live) must file for a modification of the existing order.  At that point, the court will look to many factors - home environment, income and education of the parents, the child's relationship with each parent, reason for wanting the change, etc. In other words, a minor child cannot choose which parent they want to live with. 

However, while a child cannot outright choose where they want to live, if they over the age of 12, a child's preference is given consideration and is one of the factors that the court considers (the older the child, the more weight it is given).