If you are applying to be preapproved for a mortgage, you should start to gather:
A letter of employment: Have your employer give you a letter on company letterhead outlining your name, position, gross annual income, and number of years employed with the company.
If you are self-employed, you will need three years financial statements, and tax returns (together with official assessment from Revenue Canada).
Social Insurance Numbers.
At least 3 years history of residences and employers.
Know your banking information (i.e. institutions name, address, type of accounts, account numbers)
Know your assets and their value (i.e. cash amounts, stocks, bonds, RRSPs, car).
Know your liabilities (i.e. car loan, credit card balances).
Also, be sure and advise your Mortgage Broker about any past credit problems you may have had.
Finally, write down a list of questions you would like to have answered.