You will want to start with the medical form. You can find it here on the site or at a Service Ontario center. 

There are walk-in clinics specializing in these 'medical' visits for the quick and easy cash, and they will certainly have some blank copies of the forms for you. I recommend these places not only because the visits are fast and efficient, but also, they fill out the forms many times per day, and fill them out correctly. Your family doctor may not fill out these forms usually, and they often 'miss' something. Forcing the license applicant to make another appontment, and travel to the doctor again, so they can check a box, or sign a section, they accidentally overlooked. You can find a printable form from the MENU above.

After you are medically approved to operate heavy vehicles, you should then study the M.T.O. Truck Handbook.

You will need to take the 'A' written test at any DriveTest Center. All knowledge required is in the Handbook. You may complete this step at any time, whenever you're ready, go to a DriveTest center, they will ask for the completed medical form and some money. I recommend practicing with some online practice tests first. You can find the free online M.T.O. Truck Handbook and Practice Tests in the MENU above.

Now we call Instructor Gonzo.

You will get your truck and trailer all set. Instructor Gonzo will arrive at the schedualed time, provide his self written materials, tools, and spectacular performance to teach you exactly whats required to be a Commercial Driver in the province of Ontario.