For those who, for whatever reason, have been unable to earn a high school diploma, a G.E.D. can offer many of the same benefits. According to Education-portal.com, up to 95% of U.S. colleges and universities accept a GED credential in lieu of a high school diploma as do 96% of employers.
Eligibility includes California state residency, being within 60 days of the 18th birthday, and to be able to sit for the entirety of the test. With documentation, 17 year olds may be eligible.
Test format includes: Language Arts (reading & writing), Science, Social sciences, and Mathematics. They consist of multiple-choice questions and a writing portion.
You will need to locate a GED testing center in your area and pay a fee. The overall passing score is 2250.
Some resources for taking the test can be found here.