Finding the first line
Most songs don't start with an idea. They start with a line: a phrase you overheard, a sentence in a text message, a few words that arrive with a rhythm already attached. Your first job as a songwriter is to notice those lines and keep them.
Carry a notebook, or use the notes app on your phone, and write down anything that makes you stop for a second. Don't judge it yet. A good first line usually has two things: a concrete image and a question it doesn't answer. "The porch light's still on" is more interesting than "I miss you", because the listener wants to know who left it on, and why.
When a line feels promising, sing it out loud before you write anything else. Find the natural rhythm of the words and the note it wants to land on. That rhythm will shape the whole verse, so it's worth getting right before you move on.
Verse, chorus, bridge
Structure is what lets a listener relax into your song. They know where they are, so they can pay attention to what you're saying. The most common shape in popular music is verse, chorus, verse, chorus, bridge, chorus, and it's common for a reason.
Think of the verses as the story and the chorus as the point of the story. Each verse should move us somewhere new, while the chorus stays the same and means a little more each time we hear it. The bridge is your chance to step outside the story and look at it from a different angle.
Melody that sticks
A melody sticks when it repeats just enough to be familiar and changes just enough to stay interesting. Most memorable hooks use a short phrase that repeats two or three times, with a small twist on the last repetition.
Try this: sing your chorus line three times the same way, then change only the last two notes. That small surprise is often what turns a good chorus into one people hum on the way home.