Heatherr, you really have to just follow what both your head and heart are telling you to do. At your age, you will need some help in raising the baby, and I think it's great that the father is so willing to be a part of the baby's life. It's completely fine to need help...even us "old" (lol) people need help with our kids, and need a break from time to time. Of course, if you continue contemplating adoption, that is certainly always an option also...it will be the hardest thing you'll ever have to do, but there are a lot of blessings with it also, both for you, the adoptive family, and for your baby as well. But again, you must follow your heart. Adoption is a decision that you must be 100% at peace with, even though it's a step that comes with an automatic level of self-doubt at times, I'm sure.
As far as how to divide the baby's time up between you and the father, if you decide that you do want to raise your child, he or she will definitely need you more in the beginning, particularly if you plan on breastfeeding. Even if you plan on bottlefeeding, expect as the baby's mother, to be the "default parent." That is not to say that the father's role isn't crucial, or important, or any of that. But, babies do have a tendency to bond and form a stronger attachment to their mommies, at least in the beginning, so that's something to keep in mind.
Do you have resources for acquiring what you need for the baby, if you plan on raising him/her? Brand new little newborns require very little, and as easy as it is to go overboard and buy every cute new baby item and fad on the market, they need little more than a safe place to sleep, diapers (cloth or disposable), a food source (breastmilk or formula), a few toiletries (shampoo/wash/diaper cream/lotion), and some weather and size-appropriate clothing. You won't need many toys at first, as they don't really have the ability or desire to play in the beginning...new babies do a lot of sleeping, crying, pooping, eating, and that's about it. LOL! My son is 11 weeks old, and is only JUST starting to seem interested in batting toys around a bit. What babies REALLY need to thrive are free...lots of holding, love, talking, singing, rocking, walking around, and just YOU and your attention. They are a massive time-suck, but I don't know how to describe it...they're the BEST and most rewarding time-suck on Earth.