A Social Insurance Number (SIN) is a nine-digit number you will need to start working in Canada. It is similar to a National Insurance number in the UK, a tax file number in Australia or a Personal Public Service number in Ireland.
You can apply for a SIN at a Service Canada office. There is one located in Downtown Vancouver at 757 Hastings Street West. There is one also located in Squamish on 1440 Winnipeg Street. It is often advised to go to the Vancouver office, as it is much bigger and more equipped to deal with a high volume of applicants quickly. However, the Squamish office is usually the handiest location if you are already in Whistler and need to change or update your card.
To apply you will need to bring your Working Holiday Visa and your passport. You will be given your SIN right away. Then a week or so later you will receive your SIN card in the post.