https://www.elections.ca/
But I think the online registration period may be over (do check).
That doesn't mean you can't vote on Monday though: look up the nearest polling place to your residence (ideally - you can go to a different one if you must), and go there with ID, your citizenship certificate, and something with your address (eg utility bill if your ID doesn't show your current address).* They will register you on the spot and you can vote right away. It may take a bit of extra time of course (but most will be line-ups).
It won't hurt to tell them you just became a citizen and you're voting first time. They love that.
*More than one id and more than one thing with your current address may be advisable, but take what you can. I believe they even have a process for those without proof of address but not certain what it is for federal elections (in my location's provincial elections, it involves a couple extra forms and swearing a verbal oath).
And check the elections canada website above - I think they have all this covered. I'm writing by memory and things may have changed a bit.