I am installing an additional potlight to an existing circuit in an older house. I wanted to control the potlight with an existing switch that controls other existing lights. I was able to fish a wire from the new potlight into the switch box controlling the existing lights. The existing switch box only had one set of wires. I have determined that the existing hot wire is the white wire at the switch. When I connected the wires I could not get the lights working properly. I connected the hot wire to the new light's black wire then connected the two neutrals to the switch poles. Only the new light comes on when I connected it this way. The existing lights don't work. How do I wire this properly?
There probably is no neutral at the switch box. A switch with "one set of wires" (I am guessing you mean one cable - pictures or a clear description would help me be accurate) is almost certainly a "switch loop." Under newer codes that should be a 3-wire (red-black-white-ground - ground is not counted in naming the cable, so "3-wire") cable to supply a neutral to the switch box, but under older codes a 2-wire black-white-ground cable was often used.
One wire (should be black) will be the hot to the switch. The other (should be red on the 3-wire, or on 2-wire the white wire which SHOULD have red tape or paint applied to it) will be the switched hot, from the switch, back to the first fixture. The neutral for the circuit will be at the fixture in this case.
(Pardon - North American color codes - other parts of the world do have other conventions, involving blue, brown, etc...)
Likely what you need to do is move the new light's wire to one of the other lights, not the switch.
Edit, Add - what you probably are doing is putting the one new light in series with all the other lights in parallel between it and neutral. If you remove the bulbs in all the other lights, odds are good that the new one will go out. If you remove most of them, you'll probably find that the new fixture gets dimmer, and the remaining lights on the circuit start to glow a bit...all the lights should be in parallel (from hot to neutral) but what you have now is hot, the one light, an intermediate connection from the one light to all the other lights, and through those other lights to neutral.
Yes, that is fairly simple. If you are working without drywall installed, it is a piece of cake. Easy either way though. If you wish for your add on light to work together with your existing one, simply wire black to black and white to white and bare to bare. It is simple because you already have your (wire split) that results in the single wire use on the on/off switch. Just a matter of making the circuit complete. The hard work is already done. Just did this in my shed as I wasn't happy with only the one overhead light and installed the second one in about 15 minutes. Good luck, God bless and ALWAYS be careful.