If you want to create synthesized music (that is, music that is generated by the computer and never actually played on any real instruments), you should use either
Finale Notepad or
MIDI Maestro. If you are really determined to make MIDI files on a professional level, you can purchase a MIDI keyboard to plug into your computer in harmony with one of these programs.
If you would prefer to remix sounds to create soundtracks yourself, you should try out
Acoustica or
Wave MP3 Editor.
When you have created music that you would like to put into a Starcraft map, there are plenty of programs in the SEN Download Database to convert to WAVE and to assist you with compression.
I hope this helps, good luck with your
musical mission 