The criteria is very open, like most companies. Unlike colleges and universities which impose strict criteria for admissions, companies like Google and other tech conglomerates are willing to hire employees provided they possess the necessary skill set and attitude to do the job well.
If you are an incredible coder (which you'll need to be if you want to join Google), you can always apply directly using the right channels. However, you will have to mend your profile appropriately to show you will be up for the job. One of my friends was a BA honors dropout from DU, who went on to work for Google 18 months later after working on various freelance projects. If you have the right profile and the right skills, Google will hire you irrespective of our college major.
Coming to the salary, it all comes down to how which position you're being hired for and at what location. If they want you in Palo Alto, the salary will be very high as living standards in California are considerably higher. If you're here in India, the average salary would be anywhere from 7-12 LPA. It could be more or less, but it depends entirely on what your profile says about you.