Use an ISEE test prep book, study it, do timed tests. See what you need work on, can you work on it yourself, or do you need someone to help you? If you need someone to help you, ask your parent if they can arrange some tutoring sessions for the subjects you need work on. If your parents don't mind spending the money, take a timed practice exam at a tutoring facility. Taking it somewhere other than home will help acclimate you to actually taking the test. Trade test prep books with your friends, or buy several, if you finish one. All test prep books have mistakes, so if an answer seems off, then ask your teacher.
The ISEE math only goes up to about 1st semester Algebra. There is some Geometry, but it's not like the Geometry you take after Algebra 1, it's what you learn before. At this point it's probably wiser to spend time on things you need work on, other than vocab, since there are just so many words.
Oaks Christian isn't one of the most rigorous schools, like Harvard-Westlake, Flintridge Prep or Poly Pasadena. So you don't need 90+ percentile ISEE scores across the board. The percentile score is different than a percentage score on a class test. Getting an 80 percentile means you've scored in the top 20% of test takers (a very competitive group, private school students, tougher than a typical standardized test), which many good private schools are OK with, whereas an 80% on a class test is not good, a B-. Oaks Christian gets about 600 apps for 200+ spaces, which actually isn't too bad for the LA area.
In many ways, a good GPA (from a decent middle school) is more important than your ISEE score. Anyone can study for one test like the ISEE, whereas high grades show a willingness to work consistently, and that's what they like to see. They also want to see that you will fit into the school community. When you interview, make it conversational, have a couple of questions ready. Athletics and Religion are important aspects of Oaks Christian. There are many components to admissions, the ISEE is just one part.
ISEE Website - What to Expect: http://isee.erblearn.org/guide.aspx > Select Level link on the Left.
Good luck!