Citizenship for each state is defined by the 14th Amendment to the U.S. Constitution, which states U.S. citizens are also citizens of the state "wherein they reside." To become a citizen of California, you need to establish residency. For most purposes, this isn't a complicated process. Even if you don't take specific steps, the state tax department will presume you're a resident if you live in California for nine months of a tax year. Californians can save thousands of dollars on tuition at state universities, but new California citizens may not qualify immediately for in-state tuition rates.
Establish a residence in California, either by signing a lease or buying a home. If you have children, enroll them in school. Open a bank account in California. Make an appointment with a doctor and have your medical records transferred. If you're a member of a religious organization, join a local congregation. Switch social and professional memberships to California chapters. The state tax department will also look at the address of your workplace to determine if you're a resident for tax purposes.
Visit the local office of the California Department of Motor Vehicles and get a California driver's license or state ID card. This must be done in person, not online. If you own a vehicle, you should register it in the state. California requires new residents to get a California driver's license within 10 days of moving to the state, and to transfer vehicle registrations within 20 days.
Register to vote in California. You can do this while you're at the Department of Motor Vehicles to get your new driver's license and car registration, or pick up the form at the DMV, fill it out later, and mail it to the county election office.
If you're becoming a California resident to qualify for in-state tuition at one of its state universities, you should check with the university on the specific requirements. One of the basic rules is that you must live in the state for at least 366 days to qualify, but If you're under 24 and don't have a parent living in California, it can be difficult to meet additional rules for demonstrating self-sufficiency. Check the rules; there are some exemptions.