We bought an old house to renovate and came across this situation in a bathroom and it is definitely not up to code. There is a regular outlet and a switch to lights above the mirror near the sink. There are also a couple terminals with two wires. So I want to wire this correctly using a GFCI outlet. I will have to test it to be sure, but I believe the line in is 14/2 wire to the outlet first and 14/3 wire connecting switch to lights and also connected to outlet. I am not there to see how it is exactly connected, but regardless, I want to connect it correctly. Can someone please help?? Also there is no ground with the 14/2 and it is a plastic box.
This is very tricky, because you have both a GFCI device and a switch in close proximity. It's easy to miswire it so you get an instant GFCI trip when you turn on the light.
But even before that, there's a fair chance the receptacle is already GFCI protected, and you should check for that.
If you have one of the common 3-light testers (you want the simple ones with red-yellow-yellow lights), most of them have a GFCI test button. Try it. If the power goes out, then it tripped a GFCI device somewhere else. Search for it.
You can also turn off the breaker that supplies this outlet (since after all, you'll be doing that anyway to service it, right?) First, see if that breaker is itself a GFCI breaker. If it does, your search is done. Then, search the house for which other outlets lost power also. If any of those are GFCI outlets, it's possible the bathroom has been placed "downstream of" that GFCI device, and is receiving GFCI protection from it. You would need to test further at that point; turn the circuit back on and use "Test" to trip that GFCI. If the bathroom socket loses power when you do, then your work is almost done.
If you establish that the bathroom or other plain outlets are downstream of another GFCI device, that outlet needs a sticker that says "GFCI Protected". The stickers come in boxes with GFCI devices and are also sold separately.