note:

Key

To avoid singing off key it is important to get correct the first note of the song. A major chord, say C, comprises three notes: C (root), E (third), and G (fifth). The first note of a song need not be the root note of the key chord, it can also be the third or the fifth note.

Select the key of the song. Click on each of the 3 notes of the chord to play and listen. Reproduce the tone. By singing toooo to the pitch you can check how accurate is your pitch. For checking of pitch, you need to grant permission for the browser to use the microphone. The microphone level must exceed the threshold to detect (color changed from red to green).

Sing the song starting with each of the three notes to find the one that fits with the chords of the song.

More information on the method and its limitations can be found here