Question:
I have a real urge to write a song for God I think the whole process will get me closer to him. But where do you start? Do you start with lyrics or the song itself.
Answer:
I usually sit down with my guitar... come up with a riff... find a subject... and let it flow... everybody is different... there is no set way... However, trying is a sure way to write a crappy song... let it come...
Answer:
Agreed, no set way. A few things that might help you...

• Listen to some music.
• Start playing around on the guitar, like AX mentioned.
• Read a bit of poetry.
• Just start writing words down on paper.
• Read some more poetry.

Answer:
If there is something that makes you especially admire God - Such as being in a forest, looking at the stars on a clear night, or the stained glass windows of an old Church - you can put yourself there and just think. Don't try, just think about how you admire God. Don't necessarily try to be "musical" just meditate. Something might come.