PROTIP

To find your Steam ID, log into steam from your browser and click on "Community". Your Steam ID is in the URL - it will look like either steamcommunity.com/id/{Steam ID}/home or steamcommunity.com/profiles/{Steam ID}/home.

ALTERNATIVE

Just click the "sign in through Steam" button on the home page and log in on Steam's website - you'll be returned here with your Steam ID figured out for you.