If you really want to arrange music, I suggest you should probably do a Trinity guildhall course or even ABRSM. Music arragement has its own rights, for a start you might want to do some arranging of easy peices like christmas carols. Anyway, you should have certain things in mind
- Key
- Time signature
- Length of the peice
- Any intros?
- Instruments
- Instrument specifications
- Role of each instrument
So for this, you may need certain softwares, My recommendation is to use Finale Notepad