# generate ssh key first on your source host:
ssh-keygen -t rsa

# copy it to the remote/target host:
ssh-copy-id remote_username@server_ip_addressCopy

# if for some reason the above method unavailable, use:
cat ~/.ssh/id_rsa.pub | ssh remote_username@server_ip_address "mkdir -p ~/.ssh && chmod 700 ~/.ssh && cat >> ~/.ssh/authorized_keys && chmod 600 ~/.ssh/authorized_keys"