"Won" is past tense and "win" is present "tense". The verb follows the normal tense rules. Try making two or three sentences using each tense.
Student or Learner
Do "I won" and "I win" mean the same? Are both of those used to state someone who's won a game? Does that have anything to do with timing, like "right after the game" or "a while after the game" or "a long time after the game"?
But yes, in the seconds after you win, you can say "I win!"
Five minutes later/20 years later, you must say "I won" when referring to the same contest.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
As Barb said, if your utterance is closely related in time to the victory, you can use either tense. After that, only the past works. Let's say we are playing cribbage and after I count my hand, I hit 121 points. At that moment I can say "I win". When describing the game later, I would say "I won".
Two children are playing dominoes. At the instant that one of them wins, she leaps up to her feet, skips round the room, smiling and singing/shouting "I win, I win, I win. You lose!"
An adult, hearing the noise, comes into the room, sees that the game is over and says "Who won?" The girl happily shouts "I did" or "I won!" while the other child dejectedly says "She won".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.