You're just learning so I hesitate to suggest this, but it may do the trick.

You could open a terminal "su" to root and do "dpkg-reconfigure xserver-xfree86"

That command will open up a program to set up your xserver including monitor and
resolution and everything else x does. It could take a couple of runs through it to get it right,
and if you enter the wrong parameter you could lose your xserver entirely.
That being said though it might get you the screen resolution you want.

You also could edit your /etc/X11/XFree86Config-4 check that file to
see what resolutions are listed under "monitor" Maybe editing that would work but I like the
dpkg-reconfigure method.

Good luck.