Create a user and give it a password that expires, so that they can change it when they login:

sudo useradd username
passwd username
# set password
passwd -e username

Make it a sudoer:

sudo usermod -a -G sudo username

Using SSH Keys: Source

# ensure the directory ir owned by the new user
chown -R username:username /home/username/.ssh
# make sure only the new user has permissions
chmod 700 /home/username/.ssh
chmod 600 /home/username/.ssh/authorized_keys