The boring, corporate answer is that jobs are listed under google.com/jobs, and you may get particular use from the "Students" tab on that page.
You tagged this question computer-science, so I'm going to assume that you are asking about being a Software Engineer in particular at Google, though there are various other positions. Google hires people who are right out of college, and people with industry experience (that is, experience in other companies as a software engineer) independent of a college degree.
In general, you need enough experience to be able to, on your own, go from a description of a problem to a working program that solves the problem, and to be able to understand the tradeoffs that you took making it. The actual amount of experience this will be will vary greatly from person to person. Usually, you'd get this experience by taking classes in college and high school if possible, with projects on your own, and maybe internships and jobs out of college.
Depending on where you are in your education now, there are lots of possibilities. Google has CSSI, the Computer Science Summer Institute, in a few places, for rising college freshmen to learn to program over a period of a few weeks. Then there are two kinds of internships for college students, depending on your experience. These could help you get a full job there, later.
In all cases, the most important thing you can do is not get discouraged. Even if you don't make it into something, try elsewhere, and later, if you're still interested in Google, try again! Google's interview process is notorious for sometimes missing good candidates.
I hope this helped!