Adding signals to a model railroad has got to be the most difficult part of any layout.
I found some good resources in the past to understand prototypical (i.e. real world railroad) signals are done:
This first video is over 2 hours long but well worth wading through.
This second video has follow-up videos you can find yourself.
Andy, I don't know how you did you wiring, but if you want to run more than one locomotive at a time you need to break-up the track into blocks when doing DC operation (i.e. non-DCC). On my layout, I divided the track between each turnout giving me 14 blocks. Each power driver board has a current sense circuit and returns an occupancy signal when a locomotive is in that block. The problem with this is that I can't tell where the end of the train is (but is fixable) and at first glance I should have more divisions. There's no point going into more detail since your layout is not this complex.
There is an automatic signal system out there that does not require you to have divide the track into blocks. It is independent of track power and is based on detection time. This may be a 'good enough' simulation in order to provide a little eye-candy:
https://blocksignalling.co.uk/Signal-Modules There is also this manual cheapo approach which provides signals but requires the operator to manual change switches to operate the lights. A full time job on its own, let alone operating the trains and turnouts.