I caught a whopper and I figured "Why not enter the tournament?" However, I don't know how to enter since this is my first time having a fish eligible for the tournament.
When you catch a fish it is automatically entered into the system. The top 100 fish will make it on the leaderboard. if your fish isn't on the board by midnight of every Sunday then you didn't catch a big enough fish. Tournaments are hosted weekly.
If you catch a fish that is eligible for that weeks tournament a message will pop up immediately after you caught it saying if it is eligible and if you want to enter. By saying yes it takes the fish. Like this week it is a lifecuda whopper. If you have caught one previous to this week, it doesn't count. Only the fish caught during that weekly tournament can enter. I have received 1000 gold for the fish after the week is over...a nice prize for the fish. Good luck.