No worries! From my experience it is difficult to get introductory classes, regardless of major (ex: gen chem, intro bio, physics). Upper division bio classes like immunology or animal histology are fairly easy to enroll in, as are any upper division liberal arts ones. We're talking junior-level courses and above.
However, it has been a scramble this past year for organic chemistry (there were about 50 people on the waiting list). It's becoming more and more commonplace for the university to offer only one or two lecture sessions per course.
For example, next year my CSU will only be offering two Physics A courses (one of which will be a night course). The maximum enrollment will be 50 students per course, leaving a lot of people empty-handed. To put that in perspective, over a thousand freshmen will be embarking on the science/math route next semester, leading to a pretty nasty bottleneck.
You can get priority registration by joining the honours college, which is what I did upon matriculation. It was a lifesaver. The program stipulates that you must take some designated honours courses, but it was a small price to pay for an early pick at courses. Also, if you're on a university athletics team you receive probably the best registration appointments. However, I've even been feeling the pinch despite my priority registration since my uni only allows me to enroll up to 13 units upon my registration appointment. If I want any other classes beyond my 13, I have to wait until nonrestrictive registration... which is essentially a mad dash to claim any remaining seats.
Again, this just n=1 anecdotal evidence from my large, overpopulated university (San Diego, Fullerton, Long Beach, etc). I'm assuming the situation is a lot less restricting if you're talking about one of the smaller CSUs (Channel Islands, Monterey Bay, Sonoma, etc).
Are you transferring in or are you looking to come in as a first time freshman?