A SteamID is a number used to identify individual Steam accounts

First off, a SteamID is NOT your steam account name or your player name.

A SteamID looks like this: STEAM_0:0:12345678

If you are unable to find your SteamID, here are the steps to find it.

1. Go into a MULTIPLAYER game.
2. Make sure your console is enabled by going to ESC->Options->Keyboard->Advanced->Enable Developer Console
3.Once that is enabled, press the appropriate key (usually '~' above the tab key) to open the console.
4.Type the command 'status' into the text bar.
5. Locate your name from the list of players.
6. Only copy the part that looks similar to STEAM_0:0:12345678. Do not copy the other numbers.
7. That is your steamid.