So you made a Linode account but have you actually created a new Linode server? When you first set it up, your password is whatever you set it up as in the Linode manager and your username is
root. It's up to you to create a new username.
If you've forgotten your root password, you can reset it. https://cloud.linode.com/linodes/
Once you're set up, you can use SSH from your terminal to log in. Something like
ssh firstname.lastname@example.org where
xxx is your Linode's IP address. (This can be found in the Linode's dashboard on the right hand side.)
Feel free to ask any other questions!