Fix folder and file permissions for ~/.ssh folder
SSH is checking the folder and files permissions at ~/.ssh. If there're any problems with reading or writing into that folder, the following command helps out.
chmod -R 700 ~/.ssh/ && chmod 600 ~/.ssh/*
Copy your public SSH key to the server
After submitting the following command, you will be asked for a password. Afterwards you will not be asked for the password anymore. The public SSH key will be used.