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 ...