You can start here: http://www.cic.gc.ca/english/work/
Generally speaking, to get a work permit, you first have to find an employer willing to make you a job offer; then the employer must get an approved Labour Market Opinion proving that no Canadians were available for the job; and then you would be able to apply for the work permit.
There are certain exemptions to the LMO requirement: for example, professionals applying under NAFTA or other free trade agreement, or those working in specific high demand jobs (some trades in Alberta).