The ability to successfully accompany a singer or instrumentalist on a piano largely depends on your personal experience and musical education. To create successful piano accompaniments, the pianist must have a knowledge of chords and improvisation. If you're new to accompanying performers on a piano, there are some tricks that can be learned. Fortunately we live in an age when pianists don't have to create their accompaniments from scratch. In this digital age, almost every published song or instrumental composition can be immediately printed from a computer as a lead sheet.
Request a lead sheet from the singer or the instrumentalist. A lead sheet is the basic outline of the song's melody with the chord symbols. An accompanist is expected to use the lead sheet to create a full accompaniment from this basic outline. If the singer does not have a lead sheet, you can often have one printed from your computer. An alternative to a lead sheet is a stock arrangement, which has the melody and chords plus a basic piano accompaniment.
Play the basic chord progression of the music. Don't worry about the proper accompaniment style yet. Since we're only concerned with the chord progression right now, block the chords with each hand. Blocked chords is a simple chord style where all the notes of the chord are played with each hand.
Play the chords with the melody. Play the basic chords with the left hand and melody with your right hand. Try not to double the melody notes in the chord. For example, if you see a "D" note and the chord is a "G," omit the "D" in the left hand. While you usually won't be playing the melody in performance (unless you're soloing), knowing the melody will help you determine which notes you need to leave out in order to stay out of your accompanist's way. Remember, you want to lend support to whomever you're accompanying and not overpower them.
Transpose if the key is not in the range of the singer or instrumentalist. The ability to transpose from one key to another to aid your accompanist is a required skill of all professional accompanists. Some sheet music websites allow you to print a lead sheet in any key.
Add figuration to your accompaniment. Figuration refers to arpeggios, and melodic fills that you add to make your accompaniment interesting. The figuration you add will be determined by the style of the song. If the song is a ballad, for instance, your accompaniment style could contain lots of lush arpeggios and rolled chords.