If you publish your game through Steam you don't have to provide your own update mechanism – I'd even strongly suggest not doing so. While this gives some control away from you (i.e. you have to wait for Valve to push updates), it let's you forget about all the update handling and your players also won't hit problems in case they verify Steam's cache integrity (which might cause Steam to revert the game to a previous state, which will then have to be patched once again).
Basically you just create your game with no update mechanics in place. If you publish a new version, eversyone's installation will be updated without you having to worry about it.
If you want to prevent players from using outdated versions, just include some version/protocol check when one connects online.
As for publishing on Steam, check out Steam Greenlight, if you don't have any publisher handling this for you.
It basically works like any workshop showcase. Once your game has attracted enough attention, Valve should contact you with further details and how to proceed (signing contracts and stuff).