Kittens will start to stand sometime between the third and fourth weeks, and will try to walk, although their first movements will be very wobbly. Their bodies are out of proportion to their eventual adult state. Little tails are very short and "stick-like" and their heads are disproportionately large for their bodies and legs. This will all change, though, as they get their "sea legs" and start moving around.
Don't be surprised to see kittens escaping from their nesting area, as they seek to expand their horizons. They will also interact more with their littermates, even to the point of forming "alliances" which may or may not be gender-based.
Kittens will continue to nurse regularly. It is important to continue feeding the mother a good quality kitten food, as long as she is nursing kittens.