This article is from the Care of Orphaned Kittens FAQ, posted to rec.pets.cats newsgroup. Maintained by Sharon Talbert with numerous contributions by others.
Orphans should be started on their distemper shots (done in a series of three) at six weeks. (Note: A kitten who did not receive at least the first three days of its mother's milk should be started on shots at four weeks.) The kitten should be tested for FeLV (or even FIV, if it is from a high-risk feral colony or of unknown background), and should also have its stool tested for intestinal parasites. Innoculation against FeLV (feline leukemia) will have to wait until the kitten is at least ten weeks old, but test anyway. A kitten testing positive should be held for at least two weeks (I recommend a month) and then tested a second time, to rule out a false postitive result. Starting an animal on the FeLV series without first ruling out whether the animal is a carrier is irresponsible and reprehensible!
By now your foster kitten is gobbling down kitten chow by the bowlful and drinking water on its own. That's all any weanling kitten needs, if the food is good quality and the kitten is healthy. By the time the kitten is a robust eight weeks old it is ready to go to a loving, responsible home -- if you are strong enough to let it go.
And if you do adopt out your kitten, please consider spaying or neutering it first, before it starts making kittens of its own (which it can by six months of age). A healthy kitten can be safely spayed/neutered as early as eight weeks of age (minimum weight two pounds), but at least sterilize by four months.