as this is not your SteamID, your steam ID can be found when playing games like CounterStrike/Day Of Defeat: Source
by opening your game console and typing "status" (in the previously stated games) this will list all players SteamID's
if you on a server.
The steam ID is a unique number generated by steam, when you first make an account, now what you should put in
this forums Steam ID Box is the name you use in your friends list, this way people wll be able to add you as a friend like
they would with your PSN/360 account names.
PS: Don't use your steam login name as that make's it easier for hackers to hi-jack your account. It has happened to steam
user's in the past just check the steam forums, also it is a good idea to activate the Steam Guard feature which locks your
account to your PC and also use the email Verifacation so if some one gets your user name and ties to change the PW you
get a Email tell you, at which point you cnan get steam to reset your pw through your email.