Please execute tasksel and select one of the several DM (display managers).
The job of an DM is to give a graphical screen to log in users, and, naturally, start X for that (already identified) user. Example for a Gnome Debian system.
To install a DM, debian supply a very simple (but critical) command line tool:
tasksel
If you select a version in the given list of Desktop Managers (not the same as a display manager, even if the same first two letters), the corresponding DM will be installed.

After you get several Desktop Managers installed, it could be that you want to select a diferent Display Manager even keeping the same Desktop Manager in use, because the log in screen is better for some reason. To change the default Display-Manager, execute:
dpkg-reconfigure gdm3
for the present display-manager in use.
List of available Disply Managers in Debian