how can I login to server using txx as login name and sdfg as password using su and not a root

so you don't want txx to have root privs and use `su -` to earn privs? simply create the user txx with the password you want (`adduser txx` then `passwd txx`), and change /etc/ssh/sshd_config `PermitRootLogin no` and restart sshd.