I need to attach a monitor to this computer, which cable do I need?

The port is called DMS-59. It was used to make more compact video cards, etc.

From DMS on wikipedia,

"DMS" stands for Dual Monitor Solution. "59" refers to the 59 pins. DMS-59 is generally used for computer video cards. It provides two DVI or VGA outputs in a single connector. An adapter cable is needed for conversion from DMS-59 (digital) to DVI (digital) or VGA (analog), and different types of adapter cables exist. The connector is four pins high and 15 pins wide, with a single pin missing from the bottom row, in a D-shaped shell, with thumbscrews.

Edit:

You can view or buy a compatible adapter on amazon

With some electrical ability (wiring, soldering), you could use the pinout to make your own adapter. You would need to buy a DSUB-9 to wire it as a VGA

It looks like DMS-59, which appears to provide two VGA and two DVI connectors. So using an electrical converter, your task should be achievable.

That's a DMS-59 connector. It's used on work station cards to support two monitors in one output.

Yes you can connect VGA, You will need a convertor to either twin DVI or twin VGA