which key are you having ?? the public key or the private key?? if you have been give the public key, then you need to copy that public key (id_rsa.pub) thats in your .ssh directory to the remote...
create the key, if u already have no need..
$ ssh-keygen -t rsa
copy the public key to remote servers
ssh-copy-id -i ~/.ssh/id_rsa.pub username@remote
This will prompt you for the login password...