You will need to make a slip knot from your first cast-on stitch
1) With the ball of yarn on your right, lay the end of the yarn on the palm of your left hand and hold it in place with your left thumb. With your right hand, take the yarn round your top two fingers to form a loop. Take the knitting needle through the back of the loop from right to left and use it to pick up the strand nearest to the yarn ball, as shown in the diagram. Pull the strand through to form a loop at the front.
2) Slip the yarn off your fingers leaving the loop on the needle. Gently pull on both yarn ends to tighten the knot. Then pull on the yarn leading to the ball of yarn to tighten the knot on the needle.