Obtaining Your Steam ID! (Via Status!)

Introduction To Status
The status ConCommand shows information about the server a client is connected to. The information that is shown is hostname, version, VAC security, map, the number of players in the server, and a detailed list of players. The detailed list of players includes userid, name, uniqueid, connection time, ping, loss, connection state, and IP address.

Uniqueid being your STEAM_ID:

Step 1: Join any public/private server, do not choose to Create Server. This will only give your Id as STEAM_ID_LAN.

Step 2: Open your console after you have joined a server.

Step 3: Type: status

Step 4: You now may copy the text.
It will be in the following format: STEAM_X:X:XXXXX
Example:

"MyName" STEAM_0:0:1010111 00:13 72 0 active IP_MAY_BE_HIDDEN

Console commands common to all Source-engine games.

Alternatively, you may also get your STEAM ID via www.steamidfinder.com