Have the cage ready before you pick up the rabbit. Any assembly should be done. Also have bedding ready (you don't have to put it in, just have it ready so you can quickly put it in the cage. Also have food and water dishes or bottles ready to go. A litter box is also good. Basically, you want the cage ready or only needing a few things so you can just put the rabbit in the cage once you get home. I would hold off on giving food until you get the rabbit home.
Get some food that the rabbit is used to eating. Breeders should give a small sample (usually around a weeks worth). You use this to switch the rabbit to what you want to feed. If it is a food you can get and want to use, then you can buy some yourself. It would be a good idea to have the food you want to feed available so you can start mixing right away.
Other than providing pellets and hay, I would not give other foods right away. You don't always know what the rabbit is used to eating and too much new stuff (new home, new pellets etc) can cause issues. Even if you do know, you may want to wait a couple days before giving veggies so the rabbit can settle in a bit.
Try not to bother the rabbit too much in the first few days. While what you do does depend on the rabbit, some need more time to settle in than others. Don't try too much too soon.
Make sure you know what to feed a young rabbit compared to an adult. If you are getting a baby (8 weeks to around 6 months), the diet is different than an adult (6 months and up). This is mostly the type of hay and pellets as well as the amount of pellets and if you should give veggies or not.
If the rabbit is older, ask what they are used to. The owner should have some tips on what they like and what they don't. Know they like to move the litter box around can be helpful to you. If the like a bed, that can also be good to know. Some rabbits are more destructive than others, so you may need to do some extra bunny proofing. It is also good to ask if they use a water bottle or bowl as you can then get what they use and could change it later.