Here's what you can do to make sure that the transition plan is the best for your child:
Work with your IEP team to create a plan that you agree with. Include your child in the process (if they are able).
Make sure to address your child's needs for support in all aspects of their adult life: social skills, life skills, independent living, job training, and being engaged in their community.
Give the school permission to connect you with outside agencies. You don't have to work with any agency if you don't want to, but you should see what they have to offer. You can also contact the agencies yourself.
Also see the section on Outside Agencies.