My first tournament was ENL Season 2 with ARMY. We were a siege only team and decided to try our luck in battle, which obviously didn't go very well.
Our first match resulted in a crushing defeat against swedish side FF, unfortunately nobody seems to have screenshots of this match anymore. At the time I didn't even realise we were playing in an actual tournament, I thought we were just playing some friendly matches with other teams.
The first tournament I actually perceived as such was WNL Season 1 with Deutschritter.