I'm installing some track lighting. The fixture has 3 wires, black, white and green. When I removed the cap to the electrical box on the ceiling I saw a black wire, 2 white wires connected to each other with a screw cap, and a yellow wire with a screw cap on it. The black wire runs through the box, there are no ends exposed so how do I connect that wire to the black wire on the fixture? Do I have to cut it in half? Can I assume that I can attach the white wire from the fixture to the 2 white wires in the box, and that the green wire goes to the yellow wire?

While green and yellow make pretty colors with electricity its bad to mix them. Green is your ground wire and white is the neutral. Any colored wore is likely a hot wire so test between the yellow and a white with your tester, try the wall switch if there was one to control the old fixture.