It depends on what kind of song I'm writing. For my acoustic stuff, I always write a progression first... then hum out a melody and add lyrics to fit. For my electric songs I normally do it in sort of a mixed method... Where I may have a certain hook I want to fit into the lyrics... So I'll write a riff to go behind it... Then I'll write a section of music and hum a melody that fits the overall scheme of the song. So it's different most everytime... But that's the basic formula that works for me. I think it's important to find what's the most comfortable for you... Obviously you're not comfortable so try some new things. Try to write a chord progression... all the while keep a melody in your head... Then go back and add lyrics over the top. That's the easiest way that I have come up with. But some songs just don't work that way... Especially if they're fast paced rock songs. So just experiment with those methods. Good Luck!