Most graduate students receive some sort of university funding - in the form of a graduate scholarship and/or a teaching assistantship. This page describes how that money is (typically) divided and when it is given to you. In your letter of offer you were told the total amount you will receive from the university during your tenure at UWO, and some may also see a breakdown: scholarship + TA. Not everyone receives the same pay, so this will account for most differences between yours and another student's monthly cheque (account deposit).
Pay is distributed on the second-last business day of every month, so your first pay will be late September - i.e. the end of the first month of term. Setting up direct deposit for your pay is recommended, and this can be done by accessing your My HR site on the Human Resources page. Please see the Graduate Administrator if you have questions on this.
Yes, most owe the balance of their tuition at the beginning of the first term. This is after scholarship money has been deducted and is typically in the order of a few hundred dollars. As noted above, however, the department makes every effort to provide scholarship funding to offset tuition fees.
This will vary from one student to the next. However, if you read above, you can probably make a rough calculation for budgeting purposes.
Once the tenure allotted for your degree is complete (2 years for M/A/Sc, 4 years for PhD) your funding stops, but you still must pay tuition fees - strong motivation to complete on time! Further, you cannot at that point drop to (lower-fee) part-time status, except in circumstances when you obtain a full time job. Funding also stops if you take a leave of absence (e.g., parental, medical or compassionate reasons) but then recommences when the leave is complete.