In order to find your steam ID, connect to your game server and open up your console using the ~ key.
Once the console is open, type status and hit enter.
You will then see a line similar to this: “Your Game Name” STEAM_x:x:xxxxxx
Your steamID is STEAM_x:x:xxxxxx (the x’s would be numbers in reality). Keep it safe as you may need this for various things such as enabling admin on a server.