Kittens or cats really shouldn't be having milk, certainly not cows milk as they cannot digest it properly. Some can have goats milk but it's not a necessity unless you are feeding a varied diet(i.e. RAW/BARF)which is recommended by many here. 'Kitten/Cat Milk' isn't really that good for them either and should never be substituted for water or food. Best thing is water and a healthy diet.

Have a search on here for healthy diets and good food suggestions. There are lots I have found very useful. Sometimes kittens from pet shops can be unhealthy from poor breeding or malnourishment. Best to get to your local vets and have them look over the kitten, speak to you about worming, flea treatment and vaccinations. They can also discuss diets with you but you might find from reading on here that the veterinary diets may not be what you want, which is fine but do your research before you decide.

I hate to say it but depending on the type/brand of litter you are using, it's going to be expensive. Having a kitten/cat/pet is not cheap and is a big responsibility. You need to have, depending on the size of the litter box and type of litter, 1" to 2" of litter (2.5cm to 5cm) from the bottom up. The litter needs to be cleaned(scooped) daily if not every other day and changed completely weekly (7 to 10 days give or take how dirty the cat is).