In order for you to get pre-qualified, you only need to provide some financial information to your mortgage banker. Some of the things that they will ask you for is your income, the amount of savings that you might have and any investments that you may have.
Your information will be reviewed by a lender that will let you know how much they can lend you. Once you know how much of a loan that you qualify for, you will know the range of the homes that you should be looking at.
A Pre-Qaulification is the process by which a lender will tell you how much you can afford. They do not pre-approve until they verify that the information that has been presented is accurate.
The pre-approval process will involve providing financial documents (W-2 statements, bank account statements, paycheck stubs, etc.) so that your lender can do the due diligence and verify your financial status and credit.