Welcome to the "age of majority" in California, which is 18 years old (Family Code s. 6501). You are allowed to move out of your mother's house. You are an adult and it's your choice.
Your mother is referring to a California law that obligates a parent to support an 18 year old child who is still a full-time high school student, until the child graduates or turn 19 years old, whichever comes first (Family Code s. 3901(a)). However, in CA, this does not mean that you are obligated to accept that support. You can choose to refuse assistance and move out on your own.
You are now responsible for your own actions. Make sure the first decisions your make as an adult are responsible ones. Parents often have their children's best interests at heart, and it may make more sense to wait until you graduate to move out. Have an honest discussion with your mother, adult to adult, about what is right for you at this time. In the end, your are 18, and it's your choice. Choose wisely.
With respect to your stuff, this is a complicated issue. The furniture and bed are like your mother's property. Your clothes and other personal effects are likely yours. However, this is a silly dispute and you and your mom need to sit down like adults and come to a resolution.
Mom - it's time to face the facts that your child is now an adult. You have a great opportunity to help your child transition into adulthood in a responsible and mature manner. Fighting over who owns what is silly and is a waste of time. Both of you - sit down, talk, and come up with a plan about what you are going to do over the weeks and months ahead.