Clothes should keep you warm and dry. Even on the sunniest day, it can be cool out on the water. Always take a complete change of clothes not because you will fall into the water but because you may get splashed. All good sailors carefully monitor the weather. You will not be expected to sail in conditions beyond your capabilities or wishes.
Training is normally available for beginners.
Courses range from basic boat handling to racing skills. Training can be easily modified for disabled people, and many instructors will welcome your advice on how to meet your needs. The answer to this question will vary from country to country. It's a good idea to talk to someone in your national sailing authority. Personal insurance is your choice, but clubs and boat owners should already have insurance for third party risks.