It’s just one you vibe with. Maybe a friend or family member is a fan. Maybe it’s the rival of the local area. Maybe it’s a team from the first game you watched in person. Maybe you like the mascot (if they have one).

There doesn’t have to be a rhyme or reason, it’s just a game. Find one, follow them, you become invested in them, now you’re a fan. Some people take it more seriously, some people keep it light hearted, and some people don’t care at all.