I think I might be able to help a little bit on this topic. I went from writing classical / symphonic band arrangements to more of a cliche 'rock' fashion for guitar, and all you need to realize is this: it's all the same, only arpeggiated differently. If you know your standard major/minor keys, which if you play piano shouldn't be a problem (first, third, fifth, eigth[octave first]), because it works rather similiar on the guitar. Guitar you have your power chords (first, fifth, eigth) which would be your building block right there, which is a good place to start.