A correction for the statement made by @bruingirl1001. It is not exactly true that only those who completed the UC-approved A-G courses get accepted. Many applicants attend schools without the A-G courses. Many international schools don’t have AP level or IB level courses, and some schools in the United States - such as the one I attend - offer neither AP, IB, nor UC- approved courses. Besides, homeschooled kids get in.
So, how are students who didn’t meet UC - approved course requirements qualified to be considered for admissions? They get high enough scores from SAT or ACT, and from 2 SAT Subject tests. What I mean by “high score” is that the university sets a minimum calculated value. That value is determined by SAT or ACT and SAT Subject test score. You can find the specifics of this test score calculated value on UC Admission website: http://admission.universityofcalifornia.edu/freshman/requirements/examination/index.html
Even if you are not qualified according to the calculated value, don’t lose hope. Click on the “Admission by Exception” tab and “Home-schooled students” tab on the left of the UC website (link provided above).
That being said, test scores are important. (But of course other things like extracurricular, GPA, and especially personal statements are important!!!)