I wouldn't serve milk at all. Kittens only need their mothers milk, no other milk. It's better to have them get used to drink water.
When it comes to food it's better to start off with the food the previous owner fed them. Fast switches in food can cause upset tummies and that's not a good start. However you can gradually switch them to the food you want to feed them. I recommend raw or canned food.
When it comes to litter I would start with non-clumping litter. Kittens tend to eat litter and the clumping litter may cause damage to the intestines, but you'll soon know if your kittens like no nibble at the litter or not. If they don't, go ahead and buy clumping litter the next time. I personally think clumping litter is better than non-clumping.