Depending on the officer who interviews you, they are most likely to ask you about the purpose of your trip. They'll ask if you have relatives/family there, as well as where you'll be staying. They'll ask why you're applying for a visa from the office you're applying from (i.e if you're an int'l student studying in Canada and you applied for a US visa in your country of birth, they will ask you why you did not apply while you're in Canada. This happened to me - I study in Montreal and applied for a US visa when I went home for the Holidays, and I was asked why I did not apply from Montreal's US embassy). They will ask you how long you want to stay, and what you do in Canada (work, study, etc). They'll also ask you if you've been to USA before or if you've applied for a visa before.
As long as you don't lie, have enough $, and have sufficient proof that you WILL return to Canada, you have nothing to worry about.
Hope this helps.