Download the steam SDK
export your game
unzip and place all files into the content folder (Found within the SDK folder)
open the steampipegui found in the steam SDK and put in all relevant details then upload.
then go to your steamworks dashboard and go to the app admin for your game and if you put the details in correctly in the steampipegui there should be a build you can set as default.
I know this isnt the best guide, im quite new as well, but tell me if any of it helps or where you get stuck and i will try my best to help some more.