Once you click on that button, there is a step by step process the website has in place that is easy to follow. You have to create an account to apply and once you have access to it, fill out each section. It's a long and tedious task, but this is what you have worked so hard for. So shine up those trophies of accomplishments you have succeeded at and display them proudly. Don't be humble, show the admission advisers why you should belong in the UC family.
A little piece of advice from experience...
DON'T START LATE! Seriously. Unless you like pulling all-nighters, then by all means, finish at the last moment. I had several copies of my personal statement, all of which were completely different from one another. It takes time and effort to complete the application and if you want it done right, the sooner the better.
You can submit your UC application starting from November 1 to the last day in November. But don't wait until the last day to press that submit button. Thousands of other students (who have decided to be lazy-slackers) will also be furiously pressing the submit button. Their servers can CRASH! So get on it and stay on top of it. I submitted mine mid-November which is the recommended time period to. But you can work on it beforehand of course.