Unfortunately the Free Software display drivers that most distros ship with won't give you the full range of resolutions available with most display cards. Most likely this is the problem and the way to get around it is to use the proprietary drivers for your display card.
Linux Mint introduced
MintDrivers in Linux Mint 15 to simply the task of finding and installing these drivers. You should be able to find this in the menu. With the proprietary drivers there should be another GUI configuration tool (most common are the NVidia and AMD manufacturers, each provide their own) to write a valid
Xorg.conf file with your desired settings.
If you are new to Linux, then your best bet will be to use these tools and to post another question if you run into specific difficulties. Otherwise if you add the output of the
lspci command to your question, someone here should be able to advise what to install.
As for managing an external display, there are a few options. First, if the display is always connected, you could just set this up at the same time as the
Xorg.conf is set up. However, this will cause issues when the display isn't connected. Another option is to run
xrandr when the display is plugged or unplugged. Here is an example command to set up a VGA monitor to the left of a laptop with a LVDS monitor:
xrandr --output VGA1 --auto --left-of LVDS1
There are also various GUI front ends to
xrandr. Examples are
krandr which will be available in the Linux Mint repositories (there will likely even be one installed by default).
Unfortunately, AFAIK, none of these tools are smart enough to adjust settings every time the monitor is plugged or unplugged, so the settings need to be manually applied each time. If you find this is the case, it is possible to set up a
udev rule to automatically run a script which changes the display settings. If you are interested in doing this I can add some advice on that.