Most American credit cards, including MasterCard, are widely accepted in Canada, so you do not need to worry about finding locations that accept your card. However, many card companies suggest that you notify them before using the card outside of the country. If you don't and the company notices what it considers unusual charges, it may freeze your account, at least temporarily preventing you from making purchases with the card.
Look on the back of your MasterCard for an 800 number listed there. Call the number to be connected with a customer service line. The number may differ from card to card, as some Mastercards are issued through banks and retailers. The numbers on these cards direct you to the bank's or retailer's credit card department. If you obtained the card directly from MasterCard, the phone number takes you to the official MasterCard 800 line.
State that you are traveling to Canada and provide the time frame you are traveling. It is good to contact the credit card company a week or so before traveling. Although some MasterCard providers do not require you to perform this action, it is better to be safe than sorry.
Look on the windows of retailers or restaurants in Canada for signs signifying that they accept MasterCard. If MasterCard is listed, you may purchase any product just as you would in the United States: by swiping your card through the credit card swipe and signing the given receipt.
Monitor daily exchange rates (you may do so online or through your favorite newspaper's business section). If you are purchasing something in Canada that is $25 Canadian but the exchange rate equals $23 American, you are charged the $23 American on your account (because your card is based in the United States and not Canada, you are charged in U.S. currency).