For your first question, it's difficult to answer...honestly, just write whatever comes out -- don't try to conform to creative norms. Find your own voice and roll with that.
As for what program works best, it's also difficult to answer. It's nice to have knowledge in multiple programs, as I've found it's foolish to limit yourself to one specific platform.
With that in mind, and pardon my lengthy response, you should check out Ableton Live. It's an extremely intuitive program that is FUN (key word, here). The guy who taught me Live had his mom making beats in less than a half hour -- that's how easy it is.
I'll preface my suggestion of Live with some comments on the other major platforms out there. Now, I don't mean to talk bad about these other programs...they obviously work, as people are making great music with them. But this is what I've found in working with them.
Pro Tools -- the king of audio, and the industry standard. The best program to mix in. Can be difficult to understand if you're just getting started. Sequencer is sub-standard, if you are working with MIDI.
Logic -- very powerful program, lots of virtual instruments included. EXTREMELY steep learning curve...it's a tough puppy to understand.
Digital Performer -- the best MIDI capabilities on the market. Audio is so-so, but you can make it work. A little 'blah' for my taste.
Reason -- amazing program for beats, hip-hop, etc. HUGE downside is you don't have any way to record audio (vocals, guitars, etc.) within the program.
Cubase and Sonar I haven't worked with too much, but I wasn't impressed when I used them. Sonar's interface is a friggin' mess compared to the other platforms I've used.
GarageBand -- obviously the most suggested here, and it's worth a shot if you have a Mac and don't wanna spend more money on another program. But it's the most limited compared to every other platform -- keep that in mind.
A couple suggested Sibelius, which is perfect for writing scores and the such, but for industrial rock, it won't do you any good.
So what's so good about Ableton Live?? The versatility, man. It comes with countless plug-ins already installed, you can MIDI-map them to just about any MIDI controller on the planet (I've even seen people use Guitar Hero guitars and Wii-remotes), and the Elastic Audio (the key function to Live that Pro Tools 7.4, to put it bluntly, stole) conforms every piece of audio and MIDI you're working with to a single tempo. It's also capable of being a ReWire Host or Slave, allowing other programs to simultaneously work in conjunction with it (i.e. Reason), or permitting the use of Live within another DAW, such as Pro Tools.
Basically, the way I learned it, Live is like a musical sketchpad, or canvas...and I'm not kidding when I say it practically collaborates with you in writing the music. I suggest Live to anyone who wants to write music because of this. You can experiment with the arrangement and clips (See Video: Improvising With Loops), all without even pushing 'Stop'. Creating beats is a cinch (See Video: Creating Beats). Live's interface is so unique and, again, FUN!!
I could go on more but this is already too long. I included some links to Ableton's website -- watch those videos (especially the 'Uninterrupted Creative Flow' movie), check out more, try out the free demo -- you'll see for yourself how sweet this program is.
Good luck with your music!!
I have degrees in Music, as well as Audio Production/Engineering.
Pro Tools Operator Certified.
Composing music for over ten years.