How to find SteamIDs
The SteamID is located in the url once a player has been found and selected
Steam Search: https://steamcommunity.com/search/users/
If the players SteamID from the previous link is not a number, then use this method, insert the whole steam URL from the previous URL
The SteamID to use is the numbers in steamID64
SteamID Finder: https://steamidfinder.com/
I also updated the OP
But let me also mention that generally we recommend you to just use the ingame tools to ban players where knowing the SteamID is not required. Either by looking at a player and pressing E or by right clicking on a player in the Tab menu. The only time you would need to know the actual SteamID to ban someone, would be if the player you want to ban is not currently present on the server and you therefore have to manually enter admin.command banuser 1239812093234324 into the console. This command with only supplying the ID as a parameter will also result in a permanent ban.