回複：visiting canada...

You need to apply TRV to enter Canada. You can find the form and a list of documents at:

http://www.cic.gc.ca/english/applications/visa.html

Fill the form with all the documents as well as application fee attached and send them to Canadian Embassy or Consulate to apply for your visa. You can also apply in person.

The question is that since you have applied for immigration to Canada, there is a risk that you may not get TRV for visiting purpose.

If you return to US within 30 days with a valid I-94, you don't need to apply for the US visa.