Practice by taking advantage of California's online materials. Check the resource section below to find online actual CAHSEE tests that were given in the past. The answers are given as well, so you can check your readiness for both sections. You can also find individual questions rather than entire tests, allowing you to concentrate on areas that are problematic for you.
Learn the structure of the tests, including the number of questions on each subsection of ELA and mathematics. The state offers blueprints for each section of the test. These blueprints specify exactly the types and numbers of questions from each grade level that will appear on the test. Armed with that specific knowledge, you will know exactly what to concentrate your studies on.
Familiarize yourself with the blueprints, and concentrate your studies on the areas where a lot of questions about a specific curriculum will be asked. See the resources section for links to the blueprints.
Contact your academic school counselor if you do not feel that you will be able to pass the CAHSEE. Schools are mandated to provide assistance to any student who requests it.