start by going root (aws requires you to login as ubuntu), then add your user, followed by adding the ssh key to authorized
sudo su -
adduser nate
gpasswd -a nate sudo
exit
sudo su nate
cd ~
mkdir .ssh
chmod 700 .ssh
touch .ssh/authorized_keys
chmod 600 .ssh/authorized_keys
then finally paste the ssh key into the authorized_keys file, log out and then login with the user.