The easiest and most accurate way to tune your instrument is by using an electronic tuner. An electronic tuner allows you to tune each string by indicating whether the notes are sharp (too high) or flat (too low).
There are several kinds of electronic tuners and most are relatively inexpensive and simple to operate. These are available as a standalone device from a music store or as a smartphone app.
If you have an electric instrument and a standalone tuner, you can plug into it, as you would an amplifier. If you have an acoustic instrument, use the tuner's built-in microphone instead. If using a smartphone app, we recommend Cleartune by BitCount (Android/iOS) or PolyTune by TC Electronic (iOS). There are many free options as well.