How would I file for fulll custody of my child

You would need to talk to a lawyer and bring it in front of a judge. I am sure it will be asked why? Most judges from what I have seen give joint legal (this lets both sides decide on what is best for the child) and physical to one parent or the other. For a judge to overturn a previous decision there will need to be reasons behind it as I don't think they over turn custody real easy. As in my case I have 2 kids by a previous marriage we split up and we got joint legal I got the full physical custody - but my youngest who his dad never seen when he was born took me to court for dna which it came back 99.9% he was the father. I ended up getting SOLE legal and physical custody which is different then full custody. All decisions were mine. He still has visitation rights though...

Go to your local Family court and ask for the procedure. You will need to file a petititon with the court. If the petititon will be contested then you will need an attorney to help you.

We would really need to know what state you are in and what the circumstances are to give any more specific advice than what Scott Gem said. The only exception being I would go file with the court without a lawyer to being with because there are filing, serving and timing rules with the paperwork and you could do them wrong. It isn't something you want to mess up.