The good news is that the US and Canada use the same cell phone technology, so your phone will be compatible with cell phone service in Canada. However, it is not a given that your cell phone will work in Canada, and if it does, it can be very expensive.
The flat rate plans for cell phone service usually apply only to service in the USA. Once you cross the border into Canada, you leave the coverage area of your carrier, and enter the coverage area of Canadian carriers. Although US carriers have agreements with Canadian carriers for coverage, it may be necessary for US cell phone users to set up Canadian use with their carriers. If you are on a pay-as-you-go plan, that may not be possible.
Once your account is confirmed as allowing Canadian use, the next trick is to limit the cost. Your flat monthly rate for call minutes, text, and data almost certainly does not cover use in Canada. Your calls, text, and data service in Canada will be charged by the call minute (incoming and outgoing), texts sent, and data volume. It is just like the old days of “roaming,” and like those bad old days, the charges can mount up very fast. A typical charge for calls made in Canada is 79 cents a minute. Sending text messages can be 50 cents each. If you have an iPhone or other similar device that grabs data and app information without asking, data charges can accumulate without you even knowing it. You could easily be hit with a $100 cell phone bill for a two day stay in Canada if you are not careful.
What can you do? Limit usage. Call only when necessary, and keep it short. Make each text count, and don’t get into back and forth texting conversations. Turn off all automatic data features like e-mail fetching and push notifications. Do not use any applications that get data from the phone system. To use get e-mail or use data features, take advantage of free wifi opportunities. If you connect to free wifi at a hotel or restaurant, you can turn on the data features there and get your e-mails and stuff for free. There are some flat rate plans for service and data in Canada, but those are not generally cost effective for a single short stay, and they often only reduce, but do not eliminate usage charges.
With a little attention to detail, you and your cell phone should survive Canada.