You need regular access to a piano or an electric keyboard. You cannot learn piano accompaniment unless you have a keyboard for practice. There are many brands of inexpensive electric keyboards available, many around the $200 range. Set up your keyboard on a sturdy surface with good lighting. Your fingers should always be slightly curved when you play, as if cupping an egg in your palm. This will make the chords you need for accompanying easier to play.
You will need to buy, or have another accompanist write out for you, a song chart, also known as a lead sheet, for each song you want to accompany. A song chart is a copy of a song that has the words of the song underneath the corresponding notes of the song's melody. Instead of a written-out piano accompaniment, however, a song chart has “chord symbols” written above the melody and words, at intervals. As an accompanist, you will use those chord symbols to play the right chord at the right time.
Your next step is learning to read these chord symbols. Each chord symbol spells out a specific three-note chord or triad. If the chord symbol reads “G,” that indicates the bottom note of the chord is G. If it is a “major” chord, the next two notes of the chord are B and D. If it is a G “minor” chord, the notes are B flat and D. If it is “augmented,” the notes are B and D sharp; “diminished” indicates the notes are B and D flat. Major, minor, augmented and diminished are the four main types of triads you need to start, according to the Jazz Resource Center.
In most popular music, there are three main chords which are repeated over and over again in the music. These chords are written down in the song charts. If you know these three chords—the one chord (I), the four chord (IV) and the five chord (V)—in every key, you can learn to accompany most popular songs even if you do not have a song chart for a specific song.
The one chord (I) is the first note of the key the song is written in. If the song is in C, the one chord uses the notes: C-E-G. The four chord (IV) is F—four notes up from C on the piano. It uses the notes: F-A-C. The five chord (V) is G—five notes up from C on the piano. The five chord has the notes: G-B-D. You can play these three-note chords with your left hand, your right hand or both hands together.