dude, i just picked up my old '74 xl100 from my dads house last week. been sitting about 10 years and i just started messing with it today.
i'd start with the basics:
-clean out gas tank, carb, fuel lines and petcock.
-check points for corrosion and adjustment
-new spark plug
-change oil and clean filter screen
-new battery (its 6v, so don't put a 12v charger on it!)
-check for corrosion on all battery, rectifier, and coil connections
i have the manual for it if you need some info on it. parts are cheap for these bikes, but kind of hard to come by.
actually has some parts for it, and i got some points, and forks seals from http://vmopro.com/
for cheap. there are a few companies on ebay that sell rebuild kits for the carbs, improved rectifiers, and other stuff. just need to look around for it. there are a lot of phillips head screws that get stripped out really easy, but they sell most of them at home depot. i don't know what size tires you've got, but the front on mine seems to be a bitch to find. apparently 19" tires aren't that common.