Most Canadian citizens flying to the States should remember that network providers in the US use different frequencies to those back home. If you want to use your phone over there, you'll need to double-check what frequencies it supports before deciding to purchase a sim card for USA. You can go to a mobile phone shop, or you can use the contact us link above to get in touch with us, and we would be able to check if your phone is compatible with the prepaid sim cards from T-Mobile or AT&T.
You will also need to make sure your phone is unlocked. If your phone is on a post-paid plan or contract with your local carrier it doesn't mean your phone is locked. If you are unsure, please check with your current service provider.