Choose a spot or an object on which to focus before you begin your pirouette. Keep your eye on that spot for as long as possible and, as you rotate, find that exact spot again to complete your turn in balance.
Practice standing on one leg with the help of a ballet barre. As you lift your leg, maintain your spot and shift your weight forward, as if you were to begin a rotation. Leaning back will not allow you to maintain your balance.
Tuck in your arms while doing a pirouette, as it will help you to retain speed and momentum to complete the rotation. If your body slows down during the pirouette, it is easier to lose balance and not complete the turn.