You listen to a song that you like.
You can select the melody line with your right hand.
You don’t know which chords to play with your right hand.
Usually if you know it, you will know the whole chord sequence to play. If you don’t know, you are choosing the chords by trial and error, right?
What you need to know now is to be familiar with many chord progressions. You need to know the songs that use those progressions.
For example, many Japanese animation theme songs, as well as live dramas, use the IV-V-iii-vi progression. That’s 4-5-3-6 on the main scale. You can play Yui’s Rolling Star, or TM Revolution’s Heart of Sword, and many more.
The key is to know the chord progressions of some songs, and you will already have it connected inside your head. You will be delighted and surprised to find those easily identifiable progressions in the next songs you listen to, and immediately, you will be able to play the piano!
Another chord progression is the minor progression I-III-VII-IV, or 1-3-7-4. It’s the minor scale, take note. Songs that use this include Green Day’s Boulevard of Broken Dreams, Justin Timberlake’s What Goes Around … Comes Around, and Oasis’ Wonderwall. Play the progression on the piano first, then include the right-hand melody while your left hand plays the chords.
After playing a couple of times on different keys, with different songs, you will become used to the chord progression, and later you will be able to recognize it in the songs.
The more progressions you know about, the better.
The more familiar you are with the 12 keys, the better.
The more familiar you are with all kinds of chords, the better.
The key is to keep playing.