You signed up not knowing what the benefits were? I'll bet used car salesmen love you. 🙂
http://www.cra-arc.gc.ca/tax/business/topics/gsthst/menu-e.html
That should help you out, specifically the input tax credit section. The CCRA puts out some other publications as well you'd be well advised to read.
What this means now that you're registered though is that you must *COLLECT AND REMITE* GST on *ALL* your sales to Canadian buyers. Also because you're running a business, if you're in Ontario, you must apply for a Vendor's Permit and charge and remit PST as well. For PST you'll only get credit for goods for resale and nothing else (no business supplies, etc).