When you purchase a property with existing tenants you have to assume whatever agreement the tenants had in place with the previous owner. When the initial tenancy agreement expires, the tenancy continues on a month to month basis and you cannot require the tenants to sign a new lease.

However, if the tenants do want to sign a new lease, you must use the Ontario Residential Tenancy Agreement (Standard Form of Lease), which can be found at http://www.mah.gov.on.ca/Page18704.asp