Book as early as possible! For Ontario provincial parks, reservation opens 5 months in advance. Popular parks such as Algonquin, Killbear and Killarney gets completely booked for summer and fall. If you are lucky though, there is always some last minute cancellation or the sites that can be booked on the spot, although for that, you have to arrive in the park way earlier than your office opening and hope for the best.
Planning
July and August is the best time to book as the weather is warm and less bugs. August, almost no bugs! If you are looking for northern Ontario, the bugs literally have the capacity to fly you away during spring until first week of July. Eastern parks such as Charleston has caterpillars during spring and you must consider the increasing tick infestation in Ontario, specially the west coast to east coast. Check the tick situation in your park from Ontario parks website. This link from Ontario parks gives valuable information on ticks.
https://www.ontarioparks.com/parksblog/how-to-protect-yourself-from-ticks/
Ready to book
You can book online or by phone.
Booking online for Provinicial parks - Visit https://www.ontarioparks.com/reservations
Booking online for National parks - https://www.pc.gc.ca/en/voyage-travel/reserve
Go through the legends in the campground map and familiarize yourself with the facilities that are available in that specific campground. See the location of flush toilets, showers, water tap, pit toilet, beach, hiking trails, etc. Check the pictures of your site. Find the ground cover, dimension, time of sunlight, etc.
Check the cancellation policy carefully.
Check the dimensions of the site to ensure it fits your tent, car comfortably.
If you are booking an electrical site, make sure you know the distance of the electrical outlet.
It's good to have AM sun as mornings are a little colder but if you prefer PM sun, by all means go for it.
Book a campground with partial shade. No shade would burn you off from sun and complete shade is only good for very hot days and when it's a little cold, you would miss the warmth.
Try to find a site where the water tap is close to you but not at your campsite if you enjoy privacy. You would need water frequently if you are planning to cook so find a site that has the tap closest to you although if the water tap is right at your campsite, you would continously have people coming at your site for water.
Make sure pit toilets are not very close to your site. Two problems with that - more human traffic and smell during summer months.