A must. Get the sheet music or lead sheets to as many songs as you can that you sing. Be conscious of the interval spacing of the notes as you sing and play the melody. This will automatically make you memorize the notes.
You need to practice playing the melody 100 times. Classical pianist Vladimir Horowitz would stop only after he played a song 120 times without any mistakes.
Assign a number to each note of the scale. The root of the key is a 1. The notes ascending are 2, 3, 4, 5, 6, and 7. Listen to the melody and assign the appropriate number to the notes. If the jump is an octave just say 1 to 1. There are no minus or plus numbers. This will add certainty to your singing.
Write out the melody from memory on a blank music staff sheet.
Later you add the song chords while you are also playing the lead notes and vocalizing the melody. Eventually, memorize the chords of the melody for your live singing performance.
Play chords of the song, listen while vocalizing the individual notes of the chords. Also vocalize notes of chords I, IV, V of all the keys. Vocalize the different chord types.
There are 9 different types of chords [chord quality]. In Key of C = C Major, C Minor, C dim, C aug, C Major 7th, C dominant 7, Cm7, C half diminished, C full diminished.
Play on your keyboard the song chords in two inversions so you get good at inversions and switching chords. Practice chords in both hands. You need to know the left hand rhythms and left hand note run patterns used in accompaniment. You can play the melody as an instrumental section during a break in the song that you’re singing … while playing the chords.
Do this with a metronome beat playing.