How does one craft an agreement?

So, here's the question of the hour. The mother-to-be wants to have and raise the baby. In good conscious, I can't not be a part of the child's life. Too often, I've seen young mothers with baby's the and dad is no where to be seen. I know some children who haven't had contact with their father in months or years. Unsat.

So, how does a couple -- that's really not a couple, by the way; we were both considering breaking up before there was news of the impending baby -- craft an agreement which is fair to all concerned: the child, the mother, and the father. Mom's grandmother says I should either marry or settle for occasional weekends. What sort of bullshit is that?