Hey, Im relatively new here but if you are coming to Canada you should go to Halifax, great city in the summer, lots to see and most bars per capita in Canada with lots of young people. Other good spots are Quebec City lots to see such as Chateau Frontenac, Montreal is a must, Ottawa the capital city with parliament hill is cool and the rideau canal is nice aswell as other stuff Im sure others can fill you in on. Toronto is a good town too. If you are looking for some history and Sights like Peggy's Cove, Boat tours, and that good stuff stick to Halifax and possibly Quebec city for some history too.
As of out west, I personally would focus on Alberta and British Columbia. Calgary is a good city to get started from it has the Cagary Stampede not sure the dates tho. Other places include Banff which is a blast with young people partying, skiing, etc in the summer . Jasper is much of the same. In BC the Okanagon Vally has some good sites like Kelowna for instance havent been but have heard good things, Whister is another good town. Of course Vancouver is a great city too and a must see. Lots to do, Granville street is the pace to be in the summer.
Not sure about Hostels, but check out info on these cities to get started. Just remember its a long distance from east to West So you might have to make some sacrifices.