well you don't have what you need to write songs then - unless you want to just talk over yourself playing guitar... and if that's what you like... fine. I write the melody first with the guitar part (or whatever instrument I have in my hand at the time). so I'll sing just random words while playing chords until I figure out the melody, then I'll write the lyrics. then you can add whatever instruments you want...
to be honest, this is how I started writing songs, the only way to learn how to write songs is to just do it and keep doing it. at the point where I am now, I just have a song in my head and I play it. the idea all comes out at once and then I refine it.