In order to tune a ukulele, banjo, guitar, bass etc nearly everybody will need a tuner.
You attach the tuner to the head of the instrument, pluck a string and the tuner will
register how near the sound is to a specific note, you then tighten or loosen the tension
of the string. But the simplistically clever aspect of the device is that it does not listen
to the note, it detects the speed of vibration in the wood of the head of the instrument.
Physics is wonderful.

You can get a usable tuner for as little as £6 and a decent one for £10. Of course if you are
absolutely determined to waste money you can spend £50 or more.