Welcome to Scubaboard. I live in the "Tri-Cities" and there are a few local dive shops (LDS) in the area. I will agree that Tri City Scuba is a great place. (In fact, I will be there later today to pick up my dive computer now that it has been repaired.) Both Drew & Ross are knowledgeable and will be willing and able to answer any questions that you have. If, at some point, you get into more technical diving, there is also Dean's Sport & Dive shop on Victoria Street (although it can not be seen from the road, you would drive right past it if you are heading from Guelph to Groundhog.) If you are willing to travel a little further, Dean at The Dive Academy in Oakville is also a great guy to deal with.
All of these places also offer trips, so if you want, you would be able to do your classroom and confined water locally and your open water in nice warm turquoise water.
One final thought. Later this month is the Toronto Outdoor Adventure Show at the International Centre by the Toronto Airport. There is usually a huge scuba section, and while for somebody brand new, it could become information overload, it is a great place to go and to "window shop" for both an LDS and maybe a trip.
OK, here is "one more thought" ... if you ask a couple people what agency they recommend, you will likely get a couple different answers. Go into the dive shop. Hang out for a little while. See which one you get the best "vibe" from. (Where are you the most welcome? Where will they take the time to answer your questions that they have undoubtedly heard a million times before but which are totally new for you? Which ones are more interested in guiding you, but allowing you to purchase what you want rather than which ones concentrate on selling you what they want.) Don't think of this time when you are hanging out in the dive shop as wasting time. Think of it as a very informal job interview. You are determining who you want to hire to teach you to dive, and you have to trust that person totally. As others have said, in many cases, the instructor will make more of a difference than the agency that they represent.