Pic a theme. Write the words. Something you have real feelings about will make it easier. Let the song decide what style it wants to be. Hum melodies to get the words into a rhythm then arrange it all.
music or lyrics?
write something that means something to you... people like songs that they can relate to... or that just sound cool or pretty or funny... something about something eh?
Yeah feel what you are writing inside your mind.
Say to yourself 'I;m gonna try to make a song' and relax. Start writing. Rhyme a few lines sometimes.