Hi Pmary.
Congrats to you (and to all the others with good news recently).
As has been stated, you have to drive over the border, turn around and come back IN to Canada.
Most who don't need a visa to enter the USA will stay a couple of days, do some shopping etc and then come back.
The reason for this is the border guards will (if you are literally just turning around and coming back) issue you with a refusal to enter the states and send you back to Canada. Nothing to be afraid of - just common practice.
The only reason some people advise against this is because in the future if you are ever asked if you have been refused entry into another country, you have to answer yes.
Again, its nothing too bad, but can just create some minor probems in time.