Canada does not have a typical "Au Pair" concept, where a person can come from a foreign country to work in a domestic help position in Canada. What we do have is something called the "Live-In Caregiver" program.
If your host family has a disabled child, or are responsible for an invalid parent or grandparent or something similar, they may be able to hire you as a Live-In Caregiver. Your responsibilities are pretty similar to an "au pair" position. Usually you are hired to prepare meals, look after the child, assist the invalid person, and perhaps do laundry or run errands. For that service, you generally get paid a salary, as well as are given a place to live.
You can find out more about the Live-In Caregiver program from this website.
http://www.cic.gc.ca/english/work/caregiver/apply-who.asp
The big headache is that your host family has to fill out a form called a Labour Market Opinion (LMO) stating that they tried to find someone locally to hire for that position and were unable to find a suitable candidate. You also need to have the equivalent to a Canadian high school diploma, and must have had at least six months experience as a caregiver or equivalent occupation in the last three years. So working as a nurse or nursing assistant would qualify you, for example.
You need to get the LMO, and a offer of employment from the host family BEFORE the Government of Canada will give you a work permit and let you into the country.