I added a new ceramic shower stall in bathroom also installed new can light for shower lighting. In this bathroom two lights already exist and do work. I connected 12/2 wire to new can light and ran it down the wall to wire into existing switch but can't get it to work right either the existing lights work while switch is on and the new light works when switch is off!! Also tried to add another switch lights work fine except when you turn both switches on the old stay on but the new goes off so what's up

Your new light only the black wire goes to the side of the switch that is hot when the switch is on. The white wire never touches the switch and just goes to another white neutral. Wonder why 12/2, 14/2 is quite adequate for 15 amps are is this a 20 amp circuit?

If this is a "switch loop" and has no neutral, you may be better to run to an existing light instead of the switch.
If you have 2 wires on the switch, a ground, and nothing else, it IS a switch loop.
I bet if you remove the bulbs from the 2 fixtures, the shower light won't work with switch in either position.