GO bus is pretty much an express route. There are some stops depending on where you get on and off but it's the quickest way. Depending on what time you travel, you could also take the GO train, which is probably the quickest, but it doesn't run on all routes outside of rush-hour or on weekends .
Once you get off you can either use the subway or bus. The subway network is decent, but it all depends on where you want to go.The local bus system is easy to naviagte, frequent and reliable.
I'm not sure where you want to go exactly. If your more into the tourist areas then odds are you could walk using the PATH network in downtown to get to most of the places, but the subway service downtown is excellent so you could you that too. If you want to visit people in the more suburban areas of TO then you'll need to take the subway but that almost stretches from one end of TO to the other, and from there you can easily connect to any bus route. I think there are only like 2 bus routes in TO that don't connect to a subway station and those I think are both seasonal routes to the Zoo.
Really, TO on transit is pretty easy to get around. You'll probably want to check out the TTC website and plan out your route(s) before hand though because on the spot planning can get stressful. One downside is that the TTC website is such crap and is hard to get around- trust me- the actual system is easy.