Proper diet depends on your rabbit's age. Also, when you get your new rabbit, it is very, very important that you do not make
any diet changes
whatsoever for at least several days and up to a couple weeks.
Regardless of bunny's age, you should get some of the pellets that he's been used to eating. You can use these pellets to slowly transition him to your chosen pellets (after he's had at least a few days to settle in). He should keep eating whatever he's been used to eating for about a week. (rabbits
under 6 months of age don't need their pellet amount to be limited provided they are eating plenty of hay)
Hay he should have unlimited at all times. Find out what hay he has been eating. You want him eating his body size in hay every day. You can mix a different grass hay in with what he's used to if you like.
Find out
if he's been getting fed any greens or veggies. If he has, find out what kinds, how much and how often. Greens (think more in terms of greens than veggies) need to be introduced just one type at a time and only a little bit. Suddenly giving greens to a rabbit not used to them can cause serious stomach upset. A rabbit not used to plenty of greens needs to be worked up very slowly to eating 2 cups per day.
You can read more about diet
here if you like.