If you are a small supplier, you are not required to charge your customers GST/HST and you cannot claim ITCs. However, you can choose to register voluntarily.
Once you registered, you have to charge and remit the GST/HST on your taxable supplies of goods and services, but you can claim ITCs to recover the GST/HST paid or payable on purchases related to these supplies. It is advantageous to register if the savings on ITCs outweigh the negative effect on charging customers GST/HST.
The effective date of registration is usually the date you applied to be registered. You can also specify an earlier effective date within 30 days of the date the application is received by the Canada Revenue Agency (CRA).