Heyy,
I am new to Canada and am planning to buy a bike for commuting to my workplace in the summer. My workplace is 6-7 miles away from my house. I would appreciate any suggestions and tips in buying a new or used bike in Toronto. My budget is around 400-500CA$. I'm a big time noob in terms of cycling and am looking tips from experienced people.

Thanks yo!

Some of these folks might be able to help you:

The top 5 DIY bike repair shops in Toronto

This place looks like a promising starting point:
https://www.wheelexcitement.ca

They do repairs, rentals, new sales, and sales of their rental bikes. They seem to get good reviews online.

A very-beginner of mine has been using a Trek FX 3.0 for the past couple of years, and about the same distance as you. It's relatively upright, so suitable for people who have not ridden a bike in a long time. A new one is just a tad over your budget, but I've seen used ones for around that price on Kijiji and CL in the GTA area. Might want to look into that.

You could just visit the LBS and see what they in that price range. I'm sure you can get something suitable for your first commuter.

BTW, are you able to use any of the trails around the city?

my friend has a supercycle he wants to get rid of. what about that? is it an ohk kinda bike?

You don't need $500 for a Supercycle, even a brand new one. But if he's offering it free then take it. You can ride it 6 miles. No problem.

Ride Away Bikes at Dundas and Bathurst has alot of used bike in the lower price range. I've never been there but follow them on Instagram and their prices seem to be decent. If you are looking for something a little nicer, check out My Little Bike Shop at College and Davenport. They have quite collection of nice used bikes, but call ahead because the hours are flexible.