If your employer has at least 50 employees within a 75 mile radius of your job site (as the crow flies), and assuming you have worked at least 12 months and 1,250 hours for the employer, then you are entitled to use FMLA (or the analagous California Family Rights Act (CFRA)) benefits for up to 12 weeks of unpaid
leave, without fear of being terminated from your employment.
In order to take the leave benefit, all you need to do is provide your employer with a letter requesting the leave, and stating the date when leave will begin.
Your employer is entitled to a certification from your physician that you have a "serious health condition." Many large employers have their own forms on which the certification can be provided. However, the U.S. Department of Labor has a standard form that the employer must accept, if you provide it (click here
The easiest method of handling the entire issue is to provide the letter and the certification, filled in by your physician at the same time. Make sure you keep a copy, and consider sending the letter and certification by certified mail, return receipt requested -- so you have proof that you sent it and that the employer received it.
Note: You are also entitled to apply for State Disability Insurance (SDI) benefits from the Employment Development Department (EDD) -- so as to supplement your income during leave. To apply, see this link.
Hope this helps.