I haven't already entered optometry, but I'll tackle that question anyway.
Assuming you haven't already begun college yet (and are a recent HS grad eager to get on with life), you might find it helpful to enter a major in the sciences (notably, biology). Although it's not required, many of the courses you will end up taking for your major will likely fulfill your prerequisite courses to apply to optometry school after you have graduated. While it is certainly possible to enter optometry school before your fourth year or graduation by applying for early decision, etc., the basics are the same; make plans to fulfill all the prerequisites for the schools you wish to apply to (a chart can be found here:
http://www.opted.org/info_profile.cfm), but be sure to check with each individual school to make sure you are on track and not go just by that single table.
I also recommend you attend meetings at your school's pre-optometry club (almost all the UC's will have one) to get some advice on what people in your actual school have been doing to fulfill requirements. You would get a redundant list of courses by asking what people have been taking to apply to optometry school because they would just list the pre-requisite courses listed on that table. Just plan your schedule to take these pre-requisites (be mindful of pre-requisites FOR the required pre-requisites). If you're lost with scheduling, a peer at your school may be most suitable to help you with your schedule.
I also hate to mention this, but before jumping the gun and loading up with 20 units a quarter from the start, it would also prove beneficial to try to find some experience with optometry while taking a regular course-load and enjoying "the college experience."
All in all, see if you enjoy the experience as well as evaluate whether or not more school after just three years of undergrad is really what you want to jump into. You'll have time to decide, but don't be afraid that taking a year off will be a waste of time. I know that after 4 years of undergrad at a UC, many of my peers and I would agree that a year off could be well spent relaxing, staying with family before leaving, or working to save up for another 4 years of school.
Good Luck!
Jeff
PS: Bookmark these links:
http://www.opted.org/
http://www.optstudent.com/
