The above command will output your entire public key that begins with ssh-rsa and ends with (Where USERNAME is the user name and HOST is the hostname of the machine). Once logged in, issue the command: cat ~/.ssh/id_rsa.pub To do this, log into the client machine as the user that will logging into the server. The first thing you must do is copy your public ssh key from the client machine.
What we are going to do is copy the ssh public key from the client machine to the server. Should your platforms differ, you might have to alter the instructions slightly. I will be demonstrating this on the Ubuntu Server 16.04 (server) and Elementary OS (client) platforms. I am going to assume you already have the necessary ssh key on your client and (as I already mentioned) and that you have user accounts on both client and server with the same username.
