How to buy a game on steam and generate a key?

You don't.

Seriously though, that option does not exist.

You can't buy keys from the steam store. You can however make giveaways for gifts too.

You could buy games off other websites and you get them in the form of keys that way. You cannot buy keys directly from Steam, only gift copies.

Those are games from 3rd party websites. Steam only delivers Steam Gifts unless the game can't be gifted.

You can't. You can save the game as a Steam Inventory Gift though. And then send this to someone after a giveaway.

Steam keys come from other sites, often bundle sites like Humble Bundle.

You can't..You buy from a key store..
But you can buy games from steam as gifts to give it away..

thank you guys :D
i'll make a giveaways by gift