The CRA considers that you bought the inventory at the time of your registration at its current fair market value, and you are entitled to an ITC reflecting the GST/HST that would have been paid on that value on that day. This rule does not apply to services retained prior to becoming a GST/HST registrant. Here's a link for more information:
https://quickbooks.intuit.com/ca/resources/taxes/claim-input-credit-inventory-on-hand-gst-registrant...
Registering for a GST/HST number is easy to do using CRA's online registration service. Yes, you will obtain a business number first.
On your tax return, you'll still use the usual T2125 form - just with a couple of extra steps to record the GST/HST figures from the year. You can still import all of your small business info from last year - none of that setup will change unless you decide to incorporate in the future.
