Some questions and info....
How old is the child?
With whom does the child reside with now and are there any current court orders regarding visitation or custody now?
If there are current orders, you can then file for modification of those orders by requesting "Order to Show Cause" packet. Once this is filed, in CA, they will then schedule a meeting with a mediator-
It is at this meeting that the parents try to work out whatever issues (like visitation, etc..) are ongoing-
the mediator can request to meet with the child and depending on the age of the child, can take into consideration any feelings, etc... the child might have. The mediator then will write a report and submit it to the judge.
If this is a new filing, meaning there are no current court orders in place, mediation will also occur.
I cannot stress enough the importance of this mediation meeting-the judge in my DH's case, went by the mediators recommondations almost word for word. You need to show that whatever changes you are asking for is in the best interest of the child. Also, if the mediator does request a meeting with the child, do not coach or influence that child in any way-mediators are trained to see through this.
As always, consult an attorney especially if this might be a something more complicated than what you have posted.
I am not an attorney but went through visitation issues with my DH (modification of visitation) and am currently "observing" a situation where biomom is asking for full custody and hiding child from father (new filing, still going to mediation in July).
Good luck, and hope this helps some.
Oh, and 18 is the age that a child can actually legally say where to live etc.....