To write a song
Pick a note to start your melody. The easiest way to get started writing a melody is to pick out a note to work around. Any note will do, but it’s much easier to write a flowing melody with a middle-range note, like middle C for piano or high B for guitar.
Add contrasting notes. Contrast is key to writing a great melody. You don’t have to pick notes on opposite ends of the scale, but there should be some noticeable tonal variation. Try picking a note about a quarter octave above your starting note, then bringing the next note down below the starting point. Spend some time playing with contrasting notes until you have a set that sounds good.
Use repetition to create a catchy melody. A melody by itself isn’t enough to be really memorable. Repetition is what gets a song stuck in your head. Try repeating your melody throughout your song. You can repeat it over and over again until the song ends, or only repeat it at the chorus or bridge--it’s up to you.
Change the melody a little bit throughout the song. Try changing your melody a little bit throughout your song to give it more depth and complexity. There’s no right way to do this--how you change your melody will depend on your genre and your personal preference.