To clean the metalware on your banjo you can use a very mild cleaner (eg Brasso) on all the nickel plated metalware. Just place some of the cleaner on the area you want, leave for less than a minute and wipe with a clean dry cloth.
For areas that are badly tarnished we use a product called 'Peek' to remove excess grime.
*** For Gold plated parts***
Do not use any abrasive on Gold Plated parts. Just clean with a dry cloth.
For the timber you can use a light polish like Pledge or Mr.Sheen with a dry cloth and a bit of elbow grease to help maintain the finish.
If the laquer finish is very worn in a certain area, you may need to have the effected area refinished to help protect the wood long term.