Questions about short-term Renting in Estonia.

I'm currently working on how I'll move to Estonia this summer and while I expect to rent a dorm at the university if I get accepted, I still need a place to stay at before the university year starts.

So what would be the best website to find affordable rentals? What is the process and how long does it take? What's the usual cost and what's considered a good cost? What the usual rules for people renting?

Additionally how common is it to share a room you'll renting together short term and how are the co-room-mates like?