In case it's not clear from the schedule: that route only has weekday rush hour train service (into Toronto in the morning, back out in the afternoon), but there's bus service at other times.

When you arrive in Toronto, you'll need to make your way to the Union Station GO Bus Terminal, which is across Bay Street immediately east of Union (rail) Station. There are plenty of staff in the GO concourse who can direct you.

Looks like there's one or two buses an hour at the times you'd be travelling, so keep the schedule handy.