To find your Steam ID.
1) First you need to enable you developer console; go into your options, then keyboard, advanced, enable developer console.
2) Press the "~" key on your keyboard to bring down your console window. (Under your escape key)
3) In the console window type "status".
4) You will see your Steam ID next to your name.
5) Your Steam ID will have the following format: STEAM_0:0:123456.