I found that yoyo string is made out of 2string and top string is made out of 3strings. Is there any more difference? Can I use yoyo string for the top?
The additional differences between the 2 strand and 3 strands are the ply (number of threads per strand) and the material (cotton, polyester, etc.). Yo-yo string is typically not good for tops as it is usually too thin and not long enough.
You want to look for a ball of cotton twine that you can cut for yourself. #24 cotton twine at Ace Hardware is perfect for something the size of a Bearing King. If the top is smaller or bigger you can look for something thinner or thicker. This thread tells you more about string: http://yoyoexpert.com/forums/index.php/topic,25361.0.html.
I used to make 3 ply top string, but recently started making 4 ply string. Much tighter, rounder, yeah just better.