What is the best way to respond upon knowing about someone's disability?

There is honestly no "best way to respond in such situations but you can always remember a few points :

DO NOT SYMPATHIZE UNNECESSARILY

Most often, whenever someone tells us about our disability, our first reaction is "Oh, I am so sorry". Try and avoid such statements. This reinforces the notions that persons with disability should be felt sorry for. Whenever a person opens up about their disability to you, ask them if they would like to tell you more about the nature of their disability. Always show a willingness and a readiness towards learning. Do not try to shun the topic away by saying "oh, don't worry, you'll be fine one day. Let's talk about something else."

Acknowledge their experiences. At the same time, RESPECT people if they do not want to share their unique abilities or limitations.