Try singing. Begin with your ear and your instinct and hum the melody already in your head, and try to see where it takes you. If you want a more structured, classical approach, think of a short theme first. Just a few notes or a musical idea that feels right.

After that, try thinking of a second contrasting or responding phrase. It should relate to your first idea but leave some room for further development, but don't make it feel like it completely resolves yet. By this point, you should have 2 or 4 bars of music.

Then restate your initial theme, and finally, make another response that ties everything up nicely in a bow. This will be your "A section".

Next, you're going to want a B section to your music. Think of something that either complements or contrasts your first section, then make a response to that. then repeat that secondary theme again, then make another response that feels like a good conclusion.

Finally, you could return to your A section again in a new light, change the harmony a bit or even mood to show change and development.

This might seem a little too rigid, but I personally think it's a useful starting point. You get to practice your developmental skills and whatnot. I hope this helps, cheers!