EASIEST

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.

ALTERNATIVE

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.