The first things you need to know about visiting Montreal are these:  that the city is very walkable and clean and that the Metro can take you just about anywhere you want to go. 

The Metro: Suggestion is to get a one-day, three-day, or weekly Metro pass and use it constantly.  It's a simple card system that you can use as often as you'd like within the specified period of time.  There are 4 Metro lines, all nicely connected together and easy to transfer from one line to the other with no hassle (the orange, green, blue, and yellow).  The Metro stations are located very close together and the signage is easy to read and understand.  Metro maps are everywhere, so pick one up and keep it in your pocket.  Let's say you are at Station Bonaventure and want to go to Station Champ-de Mars.  You are on the orange line.  You look at your map and see that Champ-de Mars is 3 stops away, eastward.  Rather than look for a "Champ-de Mars" sign, you look at the LAST stop on the list going that direction, which is Montmorency, and follow the signs for that stop.  When you get on the train, you get off at Champ-de Mars.  Going back to Station Bonaventure, you do the opposite --- you look for the LAST stop going in that direction, which is Cote-Vertu, and follow the signs to the appropriate spot to wait for the train.  It sounds complicated when reading this, but honestly, it is very easy to do and you will feel comfortable and confident in no time at all.  You will see on your little pocket map that the transfer stations are easy to find, and when you switch from one line to the other, you don't have to walk far at all; you pretty much just get off one train and switch to another right there.  You will not get lost!  The Metro is clean, safe, and very fast.  Trains come every 2 or 3 minutes, and you won't wait for a train for one for more than about 4-5 minutes at any time.  Each station was designed by a different architect, so they are all different and very interesting.  They are also clean and safe and don't smell bad like subway stations in many cities do.  Try taking different lines and getting off at random stops.  Even the farthest stations take only a few minutes to get to; Olympic Stadium was about 12-15 minutes from Bonaventure Station, if that.