Can i download games from Steam without downloading it just the serial then copy a friends already downloaded game from his PC onto my PC then enter my downloaded serial

No you have to download the games to play them he can come to your house log into your steam with his name but only he can play it under his name and only he can access it so both of you cant share it and play together violates their policy and is illigal copy. But what you can do is get the game and download it or ask a friend to buy the game and they have option where they can send it to your account as a gift and it will belong to you.