first: $ ssh-add private_key_openssh.txt
if the reply is something like this:

Permissions 0644 for ‘private_key_openssh_messi.txt' are too open.
this is the second part of this post :)
It is recommended that your private key files are NOT accessible by others.
This private key will be ignored.

do a chmod 0600 on the file.
ssh-add will ask you for the certificates password

second: run ssh-agent ("ssh-agent";). if it's already running, kill it (ssh-agent -k) and launch it again.

third:: simply ssh to your server :)