To apply on-line visit the School of Graduate Studies Graduate Application Page.
New students are admitted to the Computational Science Program for the Fall term which starts at the beginning of September. There is no firm application deadline, but it should be taken into account that processing each application takes several months, and, after admission, international students need to get Canadian student visas. Therefore the applications should be submitted not later than February or early March.
Moreover, financial support (if needed) is allocated on a "first-come, first-served" basis, so early applications may have some advantage. The School of Graduate Studies has information on tuition and expenses.