On the Google homepage there is a link to "Careers" where they list all their job postings, both at their physical locations and online. Usually you'll need to submit a resume, cover letter, etc. Pay attention to detail though--they get lots of applications, and if yours isn't formatted exactly as they require, it'll get tossed out without a look.
According to a recent CNN news article about who is hiring Google has several hundred applicants and only hires about 12 per day. Read the article it will help.