How long will your vacation be? East coast cruises are typically 5 to 15 days long. Cunard offers some longer trips from Southampton, UK to New England and Canada. The longer cruises visit more ports or have more sea days so, if you can be flexible on the length, you might want to go right to step 2 and pick the ports you want to visit first. Web sites like Cruise Critics list all the cruises to Canada in one place so you can compare easily.
Choose your ports of call. What cities do you particularly want to visit? Most New England and Canada cruises will stop a three or more of theses cities: Bar Harbor, ME; Boston, MA; Halifax, Nova Scotia; Montreal, Quebec; New York, NY; Newport, RI; Portland, ME; Prince Edward Island; Quebec City, Quebec; Saint John, New Brunswick; St. John's, Newfoundland. A few also include Bermuda, Charlottetown, Sydney, Nova Scotia, Norfolk and others. Again, the Cruise Critic page in the Resources section will show you where the ships stop.
As you choose your ports, you will want to visit the cruise line web sites to look at the shore excursions. Even if you don't book your on shore tours though the cruise line, these lists will give you a good idea of the kinds of activities available at each stop. Links to the Canada and New England sections of each lines web sites are below.
Choose your departure city. Do you want to leave from a particular city? Do you want to return to the departure city or is ending the cruise somewhere else OK? If you are considering spending extra time before or after the cruise, you should decide where you want to spend that time.
Check the airfare to the the various departure ports. There can be a huge difference in fares and that will make a difference in your total trip cost.
Finally, once you know the cruise length, ports of call and departure city, you are ready to choose a cruise line that offers what you want. At this step, you might have to make some compromises. Maybe no lines offers exactly what you want or the price is more than you want to pay. You will probably have to adjust you plans at this step.
Cruise lines vary from the casualness of Carnival to the luxury of Cunard. You will have to decide what style of cruising you prefer. Cruise Critics can help you with this choice too. Read some of the cruise reviews to get a feel for each.