I'm a new Norton owner myself... I found a Norton in the back of my dad's garage. It had sat for 30 years. I weaseled it out of him (At a hefty price... His only daughter and he made me pay for it!) after months of begging. The first thing I did was cleaned it out. Drained all and any fluids (Gas tank, oil tank, primary, gear box...) I removed the oil tank and stripped the inside of it with kerosene and and drywall screws. Replace oil tank rubber mounts if you take it out, the muffler mounts too. Replace the oil lines if needed, mine were still in good shape so i kept them. If i was you, i would remove the clutch plates and clean them too. You don't want them stuck together the first time you ride. You can make a clutch spring compressor pretty easy if you don't want to drop the $50 for one online. I also took the carbs apart, cleaned the crap out of them, checked to bowls to make sure they were flat and put in new needles and got a carb rebuild kit in there. New plugs, caps and wires too. Oh and in case u didn't know, these bikes can at time require special tools like witworth wrenches, clutch spring compressor, clutch lock tool, exhaust nut wrench, primary cover remover (don't use a large screw driver, it messes them up) and many more...
As said before by others, read the manual. Go over the wiring (the kill switch can sometimes be an issue) and I replaced the old points for an electronic ignition. I think the points were the reason it stopped running 30 years before.
This forum is the best place for advice. I have got a lot from these guys. It doesn't hurt to join a local Norton club too. Wish I had one close in my area. Have fun and even more, have patience. I wanted to roll my 73 850 off a cliff more then once. After six months of working on her (and a lot of money later) she is running beautifully.