The Spirit Box in Phasmophobia is provided as part of the default equipment. However, I always take a second one these days. The reason is simply because sometimes the Spirit Box will bug and not recognize voices. Having a backup provides at least an outside chance that the second one may work.
To use the Spirit Box you will need to have the lights off in the room, though flashlights and candles should not be a problem. You will also want to know if it responds only to people who are alone or to everyone. Also keep in mind that this may not matter. Trying different people alone can often yield results regardless of it saying that the ghost will respond to everyone.
You can also set the Spirit Box on the ground or a table and still speak to it and it can respond. Once you have received a response from the Spirit Box, or are sure that it won't give you a response, turn it off. Having it on can be noisy and annoying. Also, providing the ghost another possible object to interact with is a waste of time. There is also the problem that the sound of the Spirit Box in Phasmophobia is very close to the sound of Dirty Water in a Sink. This can be very confusing.
The Spirit Box seems to respond to 'Are you here?', 'Where are you?', 'How old are you?', 'Are you friendly?', and 'Hello'. There may be other things that it will respond to as well. To know if its working and your question is valid, simply watch the display on the Spirit Box. If it hears you and understands your question, either the ghost will respond or the Spirit Box will show 'Nothing Detected'.