Start at the gas tank and work toward the carb to see if you have fuel flow. The bootom of the fuel valve should have a hex head that you can unscrew to access a filter that usually needs to be cleaned out. You don't have to remove the fuel valve from the tank to clean the filter
After you clean the filter, reassemble the fuel valve and test for fuel flow in the Both the RESERVE and FULL or ON positions. If you have good flow there move onto the carb. You have to remove it to clean it properly and it has more hoses and tubes running in and out than the Space Shuttle so make sure to make a drwaing and label each hose or tube of what goes where.
The NU50 has oil injection and an automatic choke called a "ByStarter" that works off heat from the cylinder cooling fins. You should see three hoses coming from the heat sensor element on the right side of the fins going back to the carb. Label these also at the carb end so you know what goes where.
When you remove the carb, check the actual fuel intake where the fuel line connects as that can sometimes be clogged with old varnished gas.
If you need an exploded view of the carb , you can find one at http://www.partsfish.com. You have to register but it's free and then ADD a Motorcycle under HONDA and select NU50 or NU50m from the pull down list. Then select CARB from the exploded parts drawings. Order a Gasket kit for $11.99 while you are there. The bike probably won't run properly unless you replce the rubber "O" rings gaskets.
After cleaning the carb also lube your throttle cable so you don't get a run away throttle when you get it started.