How to tune your guitar (or other instrument):
If you have an electric guitar – you can connect it via Line-in input. If you have a microphone – you can use it. It is especially well suited for acoustic guitar. You can also bring a laptop (with integrated microphone) to your guitar amp.
Play a single note and watch the tuner.

Do not forget, E Standard Tuning (from thick to thin string): E, A, D, G, B, E.

If the note name is green – this string is in tune.
Cursor to the right means instrument is sharp.
Cursor to the left means instrument is flat.

How to tune your guitar without electronic tuner – you can read here: How to Tune a Guitar Without Tuner