Before you assume that a person with a disability needs your assistance, you should always ask the person if they need your assistance first. If they do, they will let you know how you can best assist them.
It is considered rude if you ask a person about their disability. If they wish to tell you about it they will and you should take their lead on what is appropriate to discuss.
Always speak directly to the person with a disability. If they use a wheelchair or if they are a little person, you should kneel down on one knee, or for extended conversations, you should sit in a chair. It’s always best to have level eye contact with the person who has the disability.
When introduced to a person with a disability, it is appropriate to offer to shake hands. People with limited hand use or who have artificial limbs can usually shake hands. (Shaking hands with the left hand is also an acceptable greeting.)
These are just my suggestions as a wheelchair user. Others may have different opinions. Remember, every person with a disability has very unique circumstances. So, my suggestions may differ from someone’ else’s. ~ Fred