I am not an expert on this kind of stuff but when I do write (though they are not a very good songs) I just play what sounds good with it just kinda play something that goes with the mood I mean if it is a sad song I would play slower and have some more lower notes like notes on on the low E, A, and D strings if it is more of an up beat song then I would play more on the G, B, and high E strings. Also something that I do is play something that sounds good with a song that is already famous and I would get inspiration from that and put it down on paper. But like I said I am not a good song writer so do what you will with my advice.