Before they turn 18 you should apply for SSI to cover some of their living expenses and medical. If they are pretty capable of functioning in a school setting you can sign them up with the DOR in your area to pay for school or help with training for a job in the community. In high school there should be a program for work program in the community during the summer to prepare them for what is to come after graduation. My son worked at a pet shop and was paid a salary. The school district will guide you with this. They also should help with mobility training for disabled students...

Make sure high school transition services are addressed prior to graduation. My adopted son never received them (he was adopted at age 22) and missed out on some good programs.

You need to make sure that both parents and, later in life siblings, have the enduring power of attorney to be able to make all the legal and medical decisions for them. I care for my 51yo brother now as mum is 82yo. Depending on circumstances, it is best to try and keep involved with groups who can provide assistance and respite care.

If you are in the United States, you should have a Transition Plan in place as part of the IEP. The school and/or district should have social workers specializing in connections to community resources and supports. Make sure that the district is doing its part in inviting post-secondary support service providers to the IEPs starting by age 16 or so. Also, your state may have one or more parent advocacy groups that can help connect you to supports, advocates, and similar resources.