can I write a song?

some one once said all you need are two chords and the truth (forgive me I can't remember where I heard it.)

Well I've got the two chords (5 actually) and I'm pretty comfortable changing between them. Still slow but that isn't the point. My point is. Knowing only what I know now how do I write chords to go with lyrics and know where to use what chord. Am I getting ahead of myself or is there something I should hear that I don't?

I've heard of people writing songs with limited guitar knowlege and I really think that if I could just have someone say "well in this song this chord is played here because... " it may help me.