This article was updated Janaury 18, 2016. 

STEP 1: SELECTING THE TYPE OF BUSINESS

The first step is to identify what type of business you want to operate, and the type of identity (sole proprietorship, partnership or corporation) that will meet your needs. A lawyer or accountant should be consulted if you are not sure which type of business best meets your needs.

STEP 2: REGISTERING YOUR BUSINESS

Registering your business in Ontario gets you a Master Business Licence (MBL), which you can use as proof of business name registration at financial institutions and in business transactions with the Ontario Government. 

ServiceOntario: Computer workstations, located throughout the province (commonly found at business help centres), which allow you to apply for Business Name Registration, Name Search, Retail Sales Tax Vendor's Permit, Employer Health Tax (including self-employed), and Workplace Safety & Insurance Board.

When to register

If you operated a sole proprietorship under your given name, ie. John Smith, you are not required to register your business.

However, operated under a modified name, such as John Smith and Associates or John Smith Designs, the Business Names Act requires that you register your sole proprietorship.

Partnerships must be registered by filing a declaration with the Companies Branch. You are responsible for determining whether your business name is already in use, which can be done by doing a name check at the ServiceOntario station.

Sole proprietorship and partnership registration do not protect your business name; others can register and use the same name. To protect your business name you need to incorporate your business, and/or apply for a trademark.

Incorporating a business is a complex process. If you plan to incorporate, consult a professional or you can research and go through the process yourself with the assistance of a good guide or book.

While it is more expensive and onerous to incorporate, a corporation may offer tax and legal benefits that make the expense worthwhile.

Where to register

Sole proprietorship or partnerships in Ontario are registered through ServiceOntario. These consist of workstations in business help centres, and registration can also be done online. Your business registration is valid for five years, after which time you need to renew your registration.

For more information and to get started on your online business registration, you can visit this website:

https://www.ontario.ca/page/business-name-registration

STEP 3: FEES

If you are completing your business registration online, you MUST have a valid email address and credit card, as this is the only way to pay for searching existing registrations as well as registering your own business.

If you wish to register in person, you can drop off completed hard-copy forms at the following address:

Ministry of Government and Consumer Services
Central Production and Verifications Branch
375 University Avenue, 2nd Floor
Toronto, ON  M5G 2M2

This location is the ONLY one that accepts in-person paper registrations and renewals. 

Here is the location website: https://www.services.gov.on.ca/locations/locationDetail.do?id=11265&_ga=1.45500772.1100351761.1450211347 

While this answer explains how to register a business in Ontario, when you register a business you also need to find out whether any regular or special licences (municipal, liquor, import-export, manufacturing, etc.) are required.

You can now register or renew an Ontario sole proprietorship or partnership register or renew a business name with the Ministry of Consumer and Business Services or apply for these accounts:

•  Ontario Retail Sales Tax
•  Ontario Employer Health Tax
•  Ontario Workplace Safety and Insurance Board

on the Canada Customs and Revenue Agency (formerly Revenue Canada) website at:www.businessregistration-inscriptionentreprise.gc.ca/

You can also register for a Business Number (GST and payroll taxes are sub-sets of the BN), and register for CCRA programs at this addresss.

* Please note that fees may change at any time; those mentioned in this answer are provided as a general guideline only to help you understand what fees to expect.