Since alot of people don't seem to know how to get this or don't know what it is, here are some ways of getting the right ID.
1. http://steamidfinder.com/ With this you can find any steamID of a player as long as you have the link to his steam profile.
2. When on any Source Engine server (CS:S, CS:GO etc), type "status" in the console and you will get a list of all the players and their steamIDs from the current server you are playing in.
If you don't know how to enable console:
In-game: g