registering to vote in Canada

I have recently moved back to Canada and decided that I ought to register to vote, can I do this online? Or do I need to go somewhere and fill out a form?

I don't know of an online process. You get on the registry of voters by either checking the box on your fed tax return every year, or more commonly, during a pre-election registration period that starts when a fed election is callled.
There is no need to pre-register if there is no election.

Thanks!