To become an admin, you first need your SteamID64.
You can find this in Steam under the details for your account (Steam -> Settings -> Account -> View Account Details).
Or on websites such as https://steamid.io.
In Basic Mode, you enter your SteamID64 under Server Settings -> Admin Settings.
In Advanced Mode, you enter your SteamID64 in the Admins.cfg file.
Please enter only one ID per line!