Probably the best way is to download the music in eg. midi format, many of the retro tunes have such available from the later fan community. (Discussed this in another thread too.)
If you are a beginner, one of the easiest to play is Usagi Yojimbo, look it up on youtube or the retro sites!
The game (originally on at least commodore 64) is based on a manga about a 17th century samurai rabbit, and you must fight off ninja, rival samurai, monsters and so on.
Put your shamisen in niagari (C-G-C) and type the scale C D F G A in eg. shamisen scale viewer from Karl. The starting/main melody is really easy when you alternate between 2 strings, it should make a lot of sense when you look at in the scale viewer.
Eg.
D-F-G – becomes --> I/2-I/4-II/0 (ichi 2, ichi 4, ni 0)
D-A-G – becomes --> I/2-II/2-II/0