There are many ways to compose songs. One of the most popular methods is to use a series of chord combinations. This basic technique enables you to write music in a wide variety of styles. Whether you're brand new to playing the piano or you've been composing your own music for years, the I-vi-IV-V chord progression offers limitless possibilities.
You can use any series of chords for the I-vi-IV-V chord progression. One of the most popular combinations is C major, A minor, F major, and G major. These familiar chords are a comfortable place for many people to start experimenting with their composition. From there, you can move on to less familiar chord combinations, such as F major, G major, E minor, and A minor, as you're ready to do so.
For now, let's focus on the C major, A minor, F major, and G major combination. You will be playing these chords with your left hand. Meanwhile you can choose notes from any of these chords for the right hand melody. For example, while you play a C major chord with your left hand, you can play E and G together with your right hand.
You may also want to think about adding a passing tone or connecting note in the melody. Instead of just playing E and G together, why not play E, F, and G in progression? The F serves as the passing note between the E and the G. As you compose, consider the rhythm. Maybe you want to play four half-beat notes, followed by one two-beat note. If you don't like how it sounds, switch it up and play two half-beat notes and then four two-beat notes. Don't be afraid to keep trying different combinations until you settle on one that you like.
Once you have a rhythm that you like, try it with the next chord. For example, switch from C major to A minor and add C and E in the right hand. Now D will serve as the passing note. Move on to play F major with A and C in the right hand, using B as the passing tone, and finally G major with B and D in the right hand, using C as the passing tone.
Now you have a base for a song. From there, you can continue to experiment with more rhythms as well as varying dynamics. Sometimes it may make sense to keep using passing notes in your right hand melodies while other times it may sound better if you stick to the notes from the chords. This technique is just one way to compose music.