The mother of my child won't let me see my child. When we both have rights. I don't know what to do? She's totally unstable and can't take care of her. I been trying to get her. I want custody but I don't know how to go about it when I don't know where she is but the town because of the mother...

Go to the court that awarded you joint custody (or whatever it is that you have) and tell them that you want your visitation enforced.

Assuming there is a court ordered visitation in place, they will ensure it is enforced. A parent cannot keep their child from the other parent, no matter how unstable they are. If she continues to defy the court, she WILL be thrown in jail and you will be granted custody.

Which isn't to say you cannot go and request custody anyway. It is just less likely since you do not currently have it.