I want full custody of my daugther?

Im 18 and was married to a lier,witch a mth later he didnt want to have anything to do with me are our 5 mth old.now hes back in town and threating to take my lil girl and he dont even want her.He wants me to sign a note under his rules but i dont know if thats legal.and hes said if i didnt sin it that he was get custody of her what do you think?