Hi Rumana! Welcome to Macinsiders! Course selection for first year science students is on Thursday June 27. However, if you want a particular elective that is really popular, try to register at 12 am on the 27th...so basically you might wanna log in to mugsi at 11 pm on the 26th
When you finish course registration that day, go to solar and click on "Payment agreement" to confirm your payment. There it will ask you if you will be paying by osap and all that. Don't forget to click "confirm" at the bottom of the payment agreement window when you finish!