Get married. First and foremost.
Here's the thing. Being that she's american, "Most" Americans are being approved in 5-7 months for clean-cut cases for OUTLAND ( clean background checks, no divorces, ect)...
If you want to apply OUTLAND - but live together, she can come here on a Visitor record and stay for 6 months. You can apply to extend that stay for another year by asking to stay while PR is being processed. However, during her time as a visitor, she cannot work. She is essentially that. A visitor.
INLAND cases take a year.. and 3-5 months just for the Open Work Permit. So that being said - please know all your options and their timeline.
If you're aplying INLAND, your Girlfriend / Wife can again come here on a visitor record. However, you need to get your App in ASAP to make sure it is accepted and she falls under "Implied Status" - this meaning she can stay in Canada during the duration of the process.. She shouldn't leave Canada at all during that time. Along with her INLAND application she can apply for an Open Work Permit - it takes 3-5 months for that to be received.
I'm in the middle of the process... An American.. I ran in to a few hiccups and would suggest that if Marriage is in your future, just cut to the chase and do it now. Just make sure you have enough proof of your relationship prior.. and try and invite a few friends and family to the ceremony. Take lots of pictures. Seriously. Document EVERYTHING. That piece of mail that someone sent addressed to both of you - keep it. It sounds silly, but seriously, document every aspect of your relationship. It makes things easier in the long run..