In root folder, there needs to be a folder that stores all the ssh keys. By default it is invisible - hence the dot before the name.
Check if the folder exists: cd .ssh
If you get a message "No such file or directory", run mkdir .ssh and go inside the folder you just made.
Create a file that will store the config touch config
Create an SSH key ssh-keygen Set file name to the service name (eg github). You can ignore the passphrase options.
Your key will be saved in service-name.pub file.
Access the generated key cat service-name.pub, copy the content and supply it to the service.
You now need to edit the config file. Open the file using your favourite editor (nano config) and add the service:
Host service-name.com
IdentityFile ~/.ssh/service-name
(When you go to Service, use settings stored in file)