Redoing an old house. All wiring 12-2. Presently have 1 old light operating to the switch. Have power there. Now, trying to run additional fixture off same existing light switch. Have tried multiple combination but have gotten no results to 2nd light fixture. Power originates in original fixture, and travels to switch. Can somebody clear this for me and possibly provide a clear picture? Do I need to pull a single strand from switch to 2nd light, and if so,
To move forward, you should really take some time and do some learning about "switch loops". Do not be experimenting blindly, understand the wiring!
You will not be doing this without either replacing one cable with /3, feeding the second lamp from the first lamp's position, or using smart switches and controllers.
You cannot fake a 12/3 by pulling a single individual wire. All related wires must travel in the same cable.
Your best bet is probably to convert all wires so they are always-hot and neutral, and then use a smart switch, which talks to two smart controllers associated with the lamp. There are also smart LED "bulbs" like the Philips HUE which provide a similar functionality; their purpose is to control colors, but they'll also work just fine for on/off.
Sometimes the power (the hot & neutral feed cable) is in the switch box and sometimes it's in the light fixture box. Either way the best place to connect for an additional light fixture controlled by the same switch is at the original fixture. That way you're sure to have both the switched hot and neutral wires needed for the new light and a better chance than the switch box of having the space for additional wires.
If the existing circuit's wire size is #14, use #14. Don't mix wire sizes. In fact never use #12 wire unless you absolutely have to. #14 wire is probably twice as easy to work with than #12, allowing you to do a much neater and safer job and it's plenty adequate for any well designed residential general lighting circuit.
The increased difficulty of working with #12 wire vs #14 for the DIY’er more often than not results in wiring of less integrity rather than more. Because now every manner of installing the larger wire becomes sloppier, it defeats any imaginary advantage of using it.