Without documentation, this is a non-trivial project and you are probably going to have to open the monitor up in order to figure out what's what. There are two basic things you need to find out first:
What voltage does the monitor require, is it AC or DC, and how much current?
Which pin(s) are power, and which pin(s) are ground.
Standard warnings about opening CRTs apply. Always assume that there are potentially life-threatening voltages in a CRT regardless of how long it has been powered off.
Once you've got those answers, you can experiment to discover the other pin assignments. The sticker on the back says 24-40V DC 1.2A, which answers the first question. Figuring out the second one is what will probably require you to open the thing up.
The individual wires in the cable will probably lead to one or two small circuit boards. If two, one board will probably be for data signals and the other board will probably be for power. If one, hopefully power and data are on distinct parts of the board. You may need to use a volt/ohm meter (preferably one with a continuity test feature) to identify which pins connect to which parts of the board.
Sidebar: If you have the thing open anyway, your best bet may be to reroute the power connections to a new jack that you install. Probably a barrel jack, but your exact choice should be dictated by what kind of replacement power supply you can come up with. A 24VDC 1.2A power supply shouldn't be hard to find, such things are commonly used to power laptops, consumer routers and various small electronics. The important thing is to find one that can supply 1.2A (finding something that can meet the 1.2A requirement will probably be harder than finding something that can meet the 24-40 VDC requirement). If you do this, and you are going to experiment to figure out the data pin assignments, then I advise disconnecting the +24V pins in the cable, so that you don't accidentally burn something out later, e.g. if by accident a pin carrying +24V touches one that's designed for only +5V (or even worse, one that's designed for 0.7V).
As for the "interfacing", once you've got the power issue settled you will again need to do some experimenting to determine which pins do what, and what kinds of signals need to be sent. There are 3 kinds of signals I would try first:
- NTSC composite output from a CGA card or an old 8-bit computer like a C64,
- "TTL" output from a CGA card (use the green pin and leave red, blue, and intensity from the CGA card disconnected), or
- Analog output from a VGA card (again, using the green pin).
There will be a lot of trial-and-error here unless you are very good at figuring out what a circuit does by looking at a board (and if you're that good you don't need any advice from me).
Once you've identified which wire carries the video signal the final step is to work out the scan frequency and refresh rate (and which pins are used for horizontal and vertical sync). Hopefully the monitor uses separate pins for HSYNC and VSYNC, and uses standard scan/refresh rates because if it's non-standard it can be very hard to work out what the 'design' rate is (and driving a monitor at the wrong rate for too long can damage it).