What is the correct way to take custody of my baby?

My fianc� of age 19 ended our 2 year relationship 3 days ago and has shown disinterest in our 7 month old baby and has not been returning home at night. She has now begun another relationship in this time and I do not feel my baby will be safe with her or around her in this emotional time. How can I gain custody of my child in Texas so she won't return home and take child and hide her from me.