General requirements for   travel documents:

Most travelers, including children, must present a valid passport to travel to foreign countries. ...
Visa-exempt foreign citizens must obtain an "Electronic Travel Authorization" (eTA) before boarding a flight to Canada.

Is it necessary to apply for eTA every time I visit Canada?

With the current eTA it is possible to travel to Canada as many times as you want for short periods of stay, which can be authorized for up to six months at each visit.
Where is the eTA processed?
On the official website of the government of Canada:
http://www.cic.gc.ca/english/visit/eta.asp
 What are the requirements to process the eTA?
Valid passport.
Credit or debit card
Email.

VACCINES

No vaccine is recommended beyond the usual ones: rabies, hepatitis B and tetanus, which are recommended even when not traveling.