TENORE PARTS

Im new in my choir NB: we are a contemporary choir and i find it very difficult to learn tenor parts for example Jesus Culture - Alleluia if i am to sing its tenor part how do i know.

ive watched alot of videos and most say i find they major key and pick the 3rd lower note

so if the song is key C the tenor would be "Mi" or "E"

but from there i get lost because i do not know how to change the other notes for it to be an actual tenor

hope i was able to explain my challenge well