Custody

So I'm due end of October, and because I'm 16 my mom wants to go for full custody of the baby, as in she wants me to have custody and I don't exactly know what that means, especially since I'm still in a relationship with the babys father. We don't currently live together but have been trying to find a place. He has expressed that he doesn't want our child because he thinks it will be too difficult but he is willing to try anyways. So would me having custody change anything if we're still together? Or does it only take effect if we break up? I'd ask a lawyer but because of my age, one won't even see me.