Technically, nothing. A good kindergarten teacher is prepared to teach a child with no knowledge of letters, sounds, or numbers, as well as one who's already reading Hemingway and working on his first novel. Obviously, though, it's ideal for a child to have a basic set of knowledge and experience when they enter kindergarten, including:
- how to correctly hold a pencil and a pair of scissors
- recognizing some or all letters of the alphabet
- counting to 10 (minimum)
- recognizing basic shapes (circle, square, triangle, rectangle) and colors
- how to sit quietly and listen to a story
- how to use the bathroom without help
Another school district has an awesome chart of Daily Activities That Prepare Children for Kindergarten; you don't necessarily have to follow this to the letter, but it has some great ideas.
Also, check out this article on What Kindergarten Teachers Wish Parents Knew.
To help your child feel less nervous, check out this list of great stories about starting kindergarten!