You can just go to any provider and get a sim card, I would recommend either Rogers/Fido, Bell/Virgin or Telus/Koodo. There's also stores like The Telephone Booth (La Cabine Téléphonique), Wireless Wave (Vague Sans-Fil) and Best Buy Mobile that can do Bell, Virgin Rogers and Fido, so you can just go to one of those and compare the different plans. There's one of each in the Eaton Centre on Ste-Catherine downtown, along with a few other local providers like Videotron (which uses a lot of the Rogers network) and Public Mobile (kind of a cheap provider, not highly recommended), so you can just go to that one shopping centre and compare the various plans and see what you like.
As for getting the SIM card, you usually just need to provide some ID. If you're getting a prepaid plan, then it's much easier than getting a post-paid plan and you won't need to pass anything like a credit check or anything like that since you're paying for the plan in advance.
Also, keep in mind that if you will be travelling my metro while you're here in Montreal, you won't get any signal in most stations or while travelling in the metro. There are a few exceptions, but overall it's a dead zone when it comes to cellular signals for now.
Edited by Symal Taneous, 10 July 2014 - 13:44.