Add the green cardamom seeds, cloves, black cardamom seeds and cinnamon sticks to a spice/coffee grinder and grind until fine. Store in an airtight container away from direct sunlight.
Most garam masalas contain cardamom, cinnamon, cloves and black peppercorns. Some folks add cumin and coriander, but I don't like the earthy flavour they add. And in fact, if you buy yours at the supermarket, it will not only contain cumin and coriander, it will also contain a bunch of other things. HOWEVER, it probably won't contain very much cardamom, cloves or cinnamon. That's because... those are the expensive ones! So what you're buying is a lot of stale filler!
I have nothing against it in a pinch, or if you're just starting out and don't know if this spice blend is for you... but if you can't find it at the supermarket, or if you're ready for the real thing, try my version. I don't use black peppercorns, but if you like, you can add a teaspoon or so. It doesn't take very long to make a batch; the only thing that's annoying about it is getting all the little black seeds out of the green cardamom pods. Use a pestle to crush the little beauties open, and discard the husks, or simply buy the seeds.
You should be able to find all of these at your friendly neighborhood Indian market, gourmet market or an online source.