in magic if you have no cards in your hand how do you discard a card?

in magic if you have no cards in your hand how do you discard a card?

You don't.

then what do you do?

Because my friend played a card asking me to discard and I didnt have any cards so he told me I lose the game, and I told him he cannot target me with that card since I am an illegal target, who is right?

Neither of you. You're a legal target, you just don't do anything, because you can't. That's really all there is to it.

Your friend may be getting confused with if you have no cards in your library and you are required to draw a card and can't you lose the game.