I am not so developed as to answer that post yet, but I was in your boat until 3 days ago and now I know lots of theory ( parents revoked my guitar and I've lots of guitar-free practice time ) and I think I can help you. First of all check out [URL=http://www.theorylessons.com] for the easiest to understand theory. Click on beginner and skip the first couple of pages about how to string the guitar, etc. He will teach you how to analyze riffs of given notes and then find the root note(tonic) and from there you can determine the song's modality and start to write leads. It's really very simple without all the fancy charts.

/by the way, I tried this on "Wherever I may roam"- metallica today and came up with B Locrian: Is that right? anybody?