The next step in performing the knitted cast on, exactly like the next step in forming a knit stitch, is to loop the yarn that is attached to the ball over the point of the right-hand needle, going counter-clockwise.
Hold the yarn loosely in your right hand as you do this. This is known as English knitting, right-hand knitting or "throwing," which is different from Continental or left-hand knitting, in which the yarn is held in the left hand.
Be careful not to loop the yarn over both needles, and don't pull too tightly or your stitches will be hard to work. Your loops should be tight enough to stay on the needle, but loose enough to slide the second needle through with ease.
This is a concept known as tension, and like in other parts of life, too much tension is bad. As you become a more experienced knitter, you'll learn what feels like the right amount of tension in your work.