What must I do right away if I take too much humulin (insulin)?

It varies. If you are alone and took more than you would normally ever take, you should call 911 because if your sugars drops too fast, you might be too weak or too confused to do so later. If someone is with you, eat simple carbs or drink juice, aiming to keep your blood sugar 200-300 until the Insulin wears off. For humulin or Novolin (insulin) R that's 6-8 hours. (see comment).