How to package my game for Steam

Hi all, pretty certain I’m not the first to ask but… how do I go about preparing and uploading my game to Steam so that millions of people can buy it and make me a millionaire? Ok perhaps one or two people.

For example do I need to get rid of all the unused assets in my game? Or does the packaging process figure out what’s used and what isn’t? Does Steam need a particular file format or collection of files from me?

Really am new to this aspect so many thanks in advance