DansTuner is a program to tell you if you are playing a pitch in tune. It

* takes audio from the default device

* if it is above a background noise level, detects the pitch

* takes the difference between that pitch and an "ideal"
(equal-tempered) pitch, and shows you that difference

* plays a guide tone along with you to help you tune

Just run it, and start playing
or singing into your computer microphone. When it detects audio above
background level, the input level will turn green, the needle will
move and the in-tune arrows will light red or green.

If you are playing in tune, the needle will be close to the center,
and the in-tune arrows will be green.

If you are not in tune, the needle will be towards the left (flat) or
towards the right (sharp), and there will be one in-tune arrow
directing you towards in-tune (the center).

Also, it plays a guide tone with you, to help you tune to it.