Working in the US
This is a link to U. S. Citizenship and Immigration Services, not one of those law firms who want to charge you a fee for something you could do yourself for free.
California Board of Registered Nursing
You can get information here about getting a license. I'm don't know how it works in Canada, but there is no national license in the U. S. Each state has its own licensing board, and you need a license for every state that you practice in. That is gradually changing, as 26 states have entered into a compact to accept one another's nursing licenses.
Necessary experience depends upon the employer. From the accounts that I have read here, the market is tight for new graduates. Gaining a year or two of experience before you relocate might be beneficial.
Most of the agencies around are travel agencies, and they generally won't take new graduates because of the degree of self sufficiency required.