Your STEAM-ID is an ID that uniquely identifies you on STEAM. It is used by stats programs to track your rank and by STEAM to store your STEAM profile.
To find out your steam ID# type status in the ingameconsole and it will be listed next to your name.
This is what will show up within the ingameconsole: # 727 "some 1337 player" STEAM_x:x:xxxxxx 01:05 112 0 active 255.255.255.255:270XX
What you want to do is to copy the STEAM_x:x:xxxxxx part.
Remember that the format is very important and you must have the STEAM_
You can also find your STEAM-ID on your stats profile page on the left pain about half way down the page under a section called Player Steamids. You can find your profile page by searching your player name in the stats.