Going to guess that you have either a CGA monitor (if it is Tandy I would guess either a CM-5 or CM-11) or a monochrome monitor (such as a VM4 or VM-5 if Tandy) and the 14 pin monitor plug is an early VGA monitor port. The two aren't really compatible.
Your easiert solution is to use a VGA monitor that has a separate cable, so you can buy a 14 pin VGA cable to attach it to this computer. A slightly more difficult method would be to use a 15 pin cable and remove the extra pin (I don't recall which one is unused; I believe the pin's use was to auto detect the resolution the computer was outputting)
A third option if you cannot find a VGA monitor is to look for a CGA or monochrome ISA card. Come to think of it, are you sure there isn't a monitor port on any of the expansion slots? Maybe someone put one in so they could use this combination at some point? I can't imagine why anyone would do that, but maybe...? But back to getting a card: I could see this option as viable only if you live somewhere that you wouldn't be able to get a VGA monitor locally, as I am sure a CGA card would be more than a VGA monitor would be at a thrift store.
A final option would be that it is quite possible that you already have an LCD TV that has a VGA port on it. Get that 14 pin VGA cable mentioned earlier and hook the computer up to that.
Hope one of these ideas work for you