Is that Canada thistle?

purpleinopp has a important point - before you till, you need to identify what weeds you've got there, as some (like Canada thistle) have roots that will send up new plants if they are cut, which a tiller will do. So, ID first, then figure out how to get rid of them.

Since "weed" killers are not available to Ontario homeowners your only option is mowing and tilling, 3 or 4 times about 2 weeks apart. At that height those "weeds" will wrap the tiller tines so they should be mowed down first. The best time to seed for a lawn in this area of the world is mid September, time for those seeds to germinate and grow and get fairly well established before winter sets in.
A soil test for pH and major nutrients would be a good idea so any adjustments that need to be made can be before seeding.

As Lisanti said, tilling thistle roots will result in more thistle plants. The other options also include smothering this whole area. Who said anything about weed killers?

I would definitely cut them down asap while you are deciding how to tackle them. At the moment they are not flowering but if you allow them to you will have a million new thistles to contend with. How big is the area? Digging them out is doable if it is not too large.