First of all, we have the music notation software. This is software based on the idea of bringing you a quick way of creating scores and parts, instead of writing everything, and then copying everything. No more copies! No more copyists! The future of music notation is upon us! Let's see which ones are the best. Primarily, we have, no big surprise here: Finale. Finale has been with us for lots of years, and has always been one of the best music software available. It will not disappoint you, it is capable of everything, though it might take you a while to get the hang of it just right. Another king of notation is Sibelius! Now, that program of easier to work with, and more friendly to the user. Its interface is kind and smooth, but on the other hand, it is not capable of as much as finale.
Another kind of music programs that we should take a look at is Cubase based software. Midi machines, in essence, capable of producing good quality pieces for those who don't have a personal orchestra to play their music. One of the best of those programs is of course Nuendo. Because Nuendo offers you the ability to use any VST program you want, which is essential if you need realistic sound, and it also provides you with a very friendly user interface, with which you can familiarize yourself pretty quickly.
Now, there is also another need of software in the public. The educational ones! The most important of those are the ear training programs. There's a variety of programs that can help you recognize intervals, harmonic progressions, chords, even train you to have a perfect pitch! An ear training program usually doesn't costs much, and it provides you with a whole lot of practical skills.
Finally, there are the audio mastering programs, that specialize in fixing the final audio file that is produced, or recorded, just before it gets in production. Those programs usually interfere with equalizing, repressing the frequencies of audio that are not needed, frequently adding some echo or delay, and enhancing the volume of the audio.
These are more or less the most prominent categories of the music software industry. Now you can research these programs, get on the internet, and find out which one you need, and which one suits you the most. Good luck!