You go to border control, tell them you're applying for a TN, hand them a copy of your identification, resume, credentials, and offer letter. Then you wait for a couple of hours while they process you, and you're set for the next few years. Rinse and repeat until your job sponsors you for an H1-B or you marry an American citizen and can apply for a Green Card.

Of course, there's other ways - talk to an immigration lawyer - but that's the simplest.