Most businesses that operate in Ontario and have sales (revenues), totaling more than $30,000 over 4 consecutive quarters must register for and charge HST. If your total annual sales are less than or equal to $30,000, you are considered to be a small supplier and are not required to register and charge HST (including International sales). Depending on the nature of your business you may still choose to register as you will be able to claim input tax credits (ITCs) to recover the GST/HST you pay or owe on your business purchases. You may also find that some clients may only do business with businesses registered for HST.
When you register for HST you are given a unique Business Number (BN). If you have not obtained a BN, you do not charge HST. Once you have registered you have to remit at the required intervals even if you have zero sales.
For further information, visit the CRA web site or call +1(800)959-5525