There are few ways to find your STEAM ID
While connected on the server
To find your Steam ID, connect to a server and then in your console type "status".
Your Steam ID is displayed beside your nickname in the format STEAM_X:X:XXXXXX.
This does not work on LAN server or while Steam is offline.
Using 3rd party websites
To find your Steam ID via 3rd party website (such as steamidfinder.com, https://steamid.xyz/), you first need to go to your steam profile.
You can do it by pressing this link: http://steamcommunity.com/my/profile
Once you click on that, you should be forwarded to your steam account. In the address bar it should look like this : http://steamcommunity.com/profiles/76561198047834xxx.
Copy that link and paste it on one of the following websites. You should get your Steam ID now.