Kind of a huge/braod question to ask. I'm not sure how drop D affects how you would write it out. Everything is in basic standard notation using the treble cleff.

Have you tried Google?
https://www.google.com/#sclient=psy-ab&hl=en&source=hp&q=how+to+write+sheet+music+for+guitar&pbx=1&oq=how+to+write+sheet+music+for+guitar&aq=f&aqi=g1g-j1&aql=&gs_sm=e&gs_upl=2089l7648l2l7865l43l18l2l11l12l1l216l2606l1.13.3l29l0&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=8174c6c347ede012&biw=1680&bih=883

The very first link I think is pretty much what you're looking for.