With all K mount lens with the diaphragm operating lever the K1000 should normally be using Open Aperture metering. It is normal for the aperture to be wide open so the view finder doesn't get dimmer as you set higher f-stops. The diaphragm should close down when you take the exposure. The needle on the exposure meter should move up or down as you change the shutter speed or aperture setting.
As mentioned before stop-down metering is used with screw-mount lenses. If there is a auto/manual lever it should be set to manual. You may have to press in the little auto/manual lock button so you can do this. This should be depressed if the lens is mount and firmly screwed into the mount/adapter.
Without film in the camera and a K mount lens attached set the shutter to "B" (bulb) and set the aperture to fully closed. Open the back of the camera and look through the lens from either end pointing it at a light colored wall or something. Press and hold down the shutter release button. You should see the iris close down. Try this with different f-stop settings. The iris opening should vary in accordance to the setting and should consistently close down to the same size as the setting when repeated. You can test this off camera too by operating the diaphragm control lever manually. If you have enough light and you are looking at the front of the lens it is not necessary to open the back. It's just difficult to see the iris from the front with some lenses.
The operating manual for the K1000 can be found here:
http://www.pentax-manuals.com/manuals/mf/pentax_k1000_manual_s.pdf
Password is "Pentax" - case-sensitive
You can read about the manual focus K-mount here:
Features and Operation of the Original K-Mount