To file any corporate tax returns or other correspondence with the CRA, you need a federal business number (or BN). When you incorporate a company, the government will automatically mail this to you. However, if you need a business number sooner, you can call CRA (1-800-959-5525) and ask for it. (As a tip when you call this number as soon as the automated voice starts if you hit the * (star) button it will direct you to someone right away instead of going through all the automated choices.)
They will ask a few questions regarding your company (the address of the company, possibly your SIN number, and they might want the incorporation number, etc). Then, they can provide the business number to you.
For more information on how to get a business number, you can see: http://www.cra-arc.gc.ca/tx/bsnss/tpcs/bn-ne/rgstr/menu-eng.htmlTags: business number, cra