I hope it wasn't iceberg lettuce. That's bad for rabbits.
I've just started taking care of a pair of rabbits myself, but I did a lot of reading up. Basically, feed your bunny alfalfa pellets, and make sure there's a good supply of alfalfa or timothy hay in the cage. fresh water. and a handful of fresh veggies once a day. Carrots, parsley, and whatever dark leaf type lettuce is in your fridge should be okay. There's a list on this forum somewhere either near the bottom of this first page or on the second page. It was posted a very short time ago.
Regular exercise and lots of love, and the baby bunny should be fine. Just lay on the floor with it and let it come to you. It's not terribly difficult.