If you are a resident of Ontario, you can operate your business as a sole proprietor. You should speak to an accountant to better understand what that will mean financially (taxes, in particular) and legally (personal litigation risks).

If a foreigner or for business reasons (taxes and litigation), you can form a company (incorporate) in Ontario.

It is best to make that decision before you start selling as doing it after the fact will result in a temporary suspension while the documents are verified.

Before you start, you need to ensure that you can legally sell the products. You need to buy from authorized (by the manufacturer/brand owner) suppliers. That will usually involve contacting them directly using contact information for their website.

It is a good idea to have a separate bank account and use a separate credit card for your business to make the accounting and tax preparation easier.

If you are going to have sales of more than $30K per year, you will need to register for GST/HST. If you form a company, that should be in the company name and you would also need to register for a business number (first), and be prepared to deal with corporate income tax. You will also usually require a business license. Check with your town/city hall regarding operating a business (from a office or from your home).

If your sales are more than $10K per month, you will also require business insurance. It is a good idea to have it even if not to address typical business risks.

You will need suitable equipment and software for your book keeping and printing documents including labels for FBA shipments. Tools of the trade including shipping boxes, tape, packing material like crumpled kraft paper.

Consult with an accountant to set things up for your book keeping and determine what you need to do and what you will pay the to do.

Dave