Using the guitar
Use your computer keyboard or click on the guitar strings to play the guitar. The four rows of keys on your keyboard correspond to four lowest strings on the guitar. Hold down the shift key and use the top two rows on your keyboard to play the two top strings.
Click "Highlight" above the guitar to show note names. Click "Sounds" to choose between different guitar sounds. Click "Mark" to mark notes on the fretboard when clicking on them. Hold down the alt key on your keyboard to mark notes with ♯ instead of ♭. Play the marked notes by clicking "Strum" or by pressing the spacebar on your keyboard.
You can save your markings on the guitar by copying the web address in your browser window. This also allows you to share your markings with others. For example, here is a C major seventh chord and an E major scale.
Online guitar tuner
Click the "Tuner" button to activate the tuner. Click the first string you want to tune and play the similar string on your guitar so that the two notes sound at the same time.
If your string is slightly out of tune, you will hear a pulsating sound between the two notes. As you tune the string, the pitches get closer and closer and the pulsation slows down. When the two notes match perfectly, the pulsation will cease and the two notes sound as one.