You would need the employer to get a labour market opinion for you so you can apply for a work permit.
If you come from a visa exempt country, that is you do not need a tourist visa, you are eligible to apply for your work permit at the border. This is the fastest way to get it. You would take your labour market opinion and $150 to the US border, cross and cross back and arrive in Canada again and get your work permit right away.
If you do not come from a visa exempt country, you are not eligible to apply at the border but if you were given a visit visa valid for 6 months or longer, you would be allowed to apply from inside Canada using internet or mail. This takes a bit longer. Looking at http://www.cic.gc.ca/english/information/times/temp.asp you see that a paper application is actually faster with 31 days, online 48 days. If you are not from a visa exempt country and they did not grant you a visit visa for 6 months, then you must leave Canada to apply for your work permit.
