I had surgery (septoplasty) and the doc told me I was a difficult intubation. What do I need to do, and what can I expect if a future surgery is need?

Be informed. The knowledge that you were difficult to intubate can be life-savinf if you need to be intubated in the future. For elective surgery, having the specific information (what techniques were attempted, what was successful, could you be mask ventilated) to give to your next anesthesiologist is the best possible way to keep safe. A "medic alert" bracelet can be useful in an emergency.
Often not modifiable. Some people are born with shorter necks and narrower upper airways. This can't be changed. But if you're above your ideal body weight, by getting down to your ideal body weight you will minimize the narrowing of your upper airway. If you require surgery in the future make sure you let the anesthetist know you were difficult to intubate so the proper equipment can be prepared.
Need more details. There are many things that can cause a difficult intubation during surgery. Ask your surgeon for more details. You should mention this to your surgeon and anesthesiologist before having any future operations.