I assume you are using the OpenSSH server.
The .ssh/authorized_keys file should be located in home direcotry of user under which you are connecting to the SSH server. So simply log in to this server under this user and edit the file ~/.ssh/authorized_keys. If the file (or directory) does not exist, create it.
The proper key format can be created e.g. using the
KeyGenerator.exe (part of Rebex SFTP samples). You can either generate the key pair there or to load an existing key there. Then you can copy the key text directly from the
KeyGenerator.exe window to the .ssh/authorized_keys file.