Pick the song lyrics. If you decided to compose the song music, you have to be ready for the text that you want to this music to put. By the way, do not forget that the text and music must somehow be in harmony with each other. Although contemporary art allows for a combination of incongruous, you must have consummate taste in music, so this combination seemed organic. So, as the lyrics of the song you can use your own work or the work of another author, but then be careful with copyright.
Read the text several times out loud, feel it and think about what associations you feel. What should be the melody to this text fast or slow? Happy or sad? What should be the musical tempo of a song? Perhaps when reading the text, your mind will be full of familiar tunes songs and classical music pieces. Listen to them for inspiration. And don't be afraid that you might accidentally "steal" a piece whose melodies for his songs – in the end, music is only seven.
Try to bring your Association to the melody. It would be good to own some instrument such as a guitar or piano. Take a few chords, try these chords to sing the text – thus is born the outline of the melody. But if you do not possess any musical instrument, then just try to sing the melody on the recorder, and then offer the familiar musician. Or contact the professionals who for a fee will do for you arranging your tunes.
Try to sing your song from beginning to end. She does not have to be the verses and the chorus but it should sound holistically, harmoniously. Record a whole song on the recorder and listen to. Maybe some places in the song cut the ears, there is a sharp transition between tunes, etc. Try to correct these mistakes.
Don't forget about the song of songs. Despite the diversity of musical styles and genres, the song should have a beginning, middle and an end. This is not recorded somewhere silly rules and peculiarities of human perception of music.