You should ask the breeder what he is used to getting. Some breeders introduce greens early on, some don’t. You don’t want to chabnge a new baby bunny’s diet to radically.
Most baby bunnies can have unlimited pellets, meaning you pour up a small bowl, then refill when you see the bowl is empty. But some buns are little piggies and will gorge themselves, and for those rabbits, you have to give a limited amount. A bunny that weighs 3,5 pounds will need about 2 tablespoons of adult pellets every day, but a baby needs more since it’s growing. There is special food for junior rabbits. Oxbow has junior pellets and so has supreme select and probably most of the other brands as well. Junior pellets have more protein than adult pellets, protein is important for growth.
In addition to pellets your bunny should have unlimited hay at all times and of course water. You can also give alfalfa hay to a baby bunny.