You might consider using neopreme washers on the nails. Ask at your local lumber yard. Don't use the old rubber style washers as they have a tendency to weather check and crack and will leak.
Hey there Shirley! PEI huh? I got relatives up there! never been and want to,,,,,any way, It's cold and windy I know in the winters. If you already have the plywood on yer roof, like the previous , put tar paper on it then put your tin down. You can get nails at the local hardware store with little gaskets already on them specifically designed for nailing down tin. Depending on what style you have of tin (hips and valleys) you can just nail the tin down where it's flat against the plywood. The nail gaskets will seal it great! I put a tin roof on my shed, and just gooped up the holes with silicone. works great, and it's lasted for 11 years with no leaks! Have fun!
I do not have any plywood under my tin roof and I used screws with rubber or neopreme washers. I set my drill to snug them but not flatten out the washers. If you flatten them too much they will leak eventually. We just built a new barn also made of metal and did the same thing.