What address do I use to register for information on the GST & HST?
It's not exactly clear what you're asking, but:
(1) If you're interested in learning about registering for the GST or HST – for instance, if you run a small business or work as an independent contractor and need to charge the tax – then please check out Canada Revenue Agency - Registering (opening) your GST/HST account.
(2) If you're interested instead in more information on the transition to HST in Ontario, you'll find some information on the move to HST at:
Ontario Ministry of Revenue:
Harmonized Sales Tax (HST) and Comprehensive Tax Package.
This information is a good starting point from the consumer perspective.
There's also some great information on the HST at TaxTips.ca - Ontario HST.
If there's something more specific you're interested to know about the GST or HST, please comment or revise your question to let us know.