You will be considered on your personal qualities and skills such as your character, ability to lead, creativity as expressed in different interests, and your intellectual curiosity which should be reflected in the classes and activities you were involved with in high school. Keep in mind that UCLA doesn't just look at your academic record, but will also consider your personal statement and your extracurricular activities when making a decision.
UCLA's admission department will look at your grades and consider the difficulty of your classes and the grades you received. Senior year is especially important, as it demonstrates the ways in which you were preparing for college. Any advanced placement (AP) and honors classes you took will give you an advantage because it shows that you pushed yourself academically and are ready to take on the challenges of a college curriculum. You should also make sure that you've completed the core courses that UCLA requires as a prerequisite for admission. The courses include two years in history/social science; four years of English; three years of math; two years of lab science; a year of visual and performing arts; two years of foreign language and a college preparatory elective.
Your scores on standardized tests such as the SAT or ACT will be considered. Though UCLA requires you to have taken these tests, there is no minimum score threshold as these tests are used as only one part of your evaluation, and are not given more or less weight than other aspects of your application.
The strength of your high school curriculum will be considered in context of how well you took advantage of the opportunities that were available in your school. For example, AP, honors and college preparatory classes are looked upon favorably, though you won't be penalized if those kinds of opportunities weren't available at your school. In some cases, your application may be compared to the application of another student who attended your high school and took the same classes in order to evaluate both of you based on similar opportunities.
If you faced hardship or other difficulties in high school, your application will be considered based on how well you overcame those challenges as demonstrated by your GPA, how well your grades improved each year and your community involvement. In addition, UCLA is a diverse university and is always looking to admit students who bring different cultural backgrounds to campus. You should spend time on your personal statement to make sure it documents your upbringing, your interests and the activities and memberships that make you a unique individual.