The moment you turned 18 you became an adult. You can do what you want.
However, if the car is in their name, it's theirs and they do not have to give it to you.
The furniture in the house is also theirs and they don't have to let you take it.
You can also talk to your school about their policy. For example, in the Dallas area, my nephew signed his own report cards after he turns 18. Here in Houston, I signed my daughter's report cards even after she turned 18. Each school district has different policies. I believe as an attorney that Dallas was correct & HISD was wrong.
But, as a parent, I liked HISD policy.