Hello Sierrra1
You have bit to learn about electrics/electronics I have been at for almost 70 years, a man next door was amateur radio enthusiast and got me going when i was six years old 
. I hope you find such a good electronics buddy, plus there is this forum and lots of help here. Ground is Ground is Ground, so just make up some type of Y wire cable to give you the Ground for one end of your switch. The other end of your switch goes to the input port you intend to use, you can use the port in digital or analogue mode, the general convention is to use the port in digital mode. Then you need to pull the Switch/Input Pin to 5V this can be done with a resistor pretty much anything from 1k to 10k or more will be fine.
Or some Digital input ports can have an internal Pull Resistor that you turn on like so from software pinMode(2, INPUT_PULLUP);
Hope that helps imk
PS the solution above of using one digital output port to drag one end of the switch low is ok but it is not Ground. It will work but not ground only close to it. But it is a very useful technique if you have MORE switches than inputs as you can create a matrix of switches and then use some digital pins to pull a row/column low/ground with a scanning technique. This is bit beyond you i think now but bare it in mind for a future learning experience
PPS One final word on Ground is, it is worth reading up on the subject with respect to Ground loops that can be most troublesome at times particularly in analogue/audio circuits as they can lead to noise