I want to start doing this, not like James Blunt cause that just crap but a bit like Jack Johnson but with a bit more "umph"! The lyrics i'm ok with, but i'm finding the music quite hard to write. Can anyone help?
It helps for me to keep the music in mind while I'm writing and have some general idea of a chord progression for it or at the minimum a rhythm for the song that I can fit chords on top of afterwards. Sometimes I change lyric I have already written as well in order to fit the pattern of the song.
Chord progression first, vocal melody second, lyrics third.

That's how I do it... although sometimes I come up with a melody first (usually on guitar) and then put chords to it, but that's only happened a few times.
What part of the music writing are you having trouble with? One suggestion would be to analyze the chord progressions of a lot of songs you like. Look for chord changes that appeal to you and play around with making them your own.
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chords first with a rhythm and then over that just simply hum a vocal line and then write lyrics to fit with the guitar...simple
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mmm... it's quite personal. I first write a short melody, then strum some chords, and when it fits it, extend the melody