If you have a game such as Counterstrike 1.6 or Counterstrike Source, I know in there you can open your console and type "Status" and it'll list the people's steam ID's and you copy yours.
ok so if u follow the place where ure steam folder is there is a Notepad File labeled Steam it shows some logs, all you have to do is put STEAM_ dont forget the underscore BEFORE the numbers Circled.