I am making a musical instrument from a raspberry pi out of cardboard. It would be like a piano with an octave of notes (13 including sharps). How would I get the raspberry pi to detect when a circuit is made. Would I use the GPIO? Is there enough, as I also want to add a few LEDs, for information like wifi connection (it would be running headless), and modular keys, like one to trigger a drum sound. I don't know much about GPIO, like which pins I can use for input. I am writing the script in NodeJS, so I assume I would use something like https://www.npmjs.com/package/rpi-gpio. Also, I intend for this to be a cheap device (hopefully under £30 GBP/$38 USD), so it would use the pi 0W.
All modern Pi's with the 40-pin header (including the 0W) have 26 GPIO pins - more than enough for 13 switches and some LEDs. Each one can either be an input or an output.
For the keys, you could use some standard, run-of-the-mill tactile pushbuttons. You can find a tutorial on the concepts behind this here (although it's for Python, not Node). Drum triggers would also be pretty simple; just hook up a mono jack in place of the pushbutton.
I'd recommend looking through some basic GPIO tutorials first for the Pi, to help you grasp how they work, such as this one on W3Schools.