I am a big noob, sorry. :(


Deauthorize all other devices here:
https://steamproxy.com/steamstore/twofactor/manage
Revoke Steam web api key here:
https://steamproxy.com/dev/apikey
(you do not need to set up a new one)
Check if there are unwanted logins here:
https://steamproxy.com/steamhelp/accountdata/SteamLoginHistory
Also change your Steam Password !!
------------------------------------------------------------------------------------------