1. A knowledge of piano would be very helpful, though not absolutely required. It's useful for several reasons: you have plenty of octaves to play with, you can work on melody and harmony at the same time, and probably some other reasons I can't think of at the same time.
2. You absolutely do not have to know how to play all instruments. You will ultimately have to learn about orchestration, which saves you that trouble.
3. You're not too young to be composing music. I'm not really sure why you're asking that question.
4. Several things will affect the number of people who buy your music: luck, publicity, quality...music is a very competitive field. Majoring in music is by no means a guarantee that anyone will buy your music.
5. You start by learning music theory. Some people say that learning music theory will kill your creativity. I would recommend that those people go live in houses designed and built by people with no knowledge of physics or geometry. Get a book on music theory -ask a teacher to recommend one for you- and start working through it. No point in reinventing the wheel.
Regards the actual process of composition...well, I only know what I do, what my dad did and does -he was a professional musician during the 80s- and what my composer friend does. Go to your instrument. Play something. If you like it, write it down. Now mess around some more until you find something else you like, but try to incorporate some aspect of your original tune, like a part of the tune or the rhythm of it. Doesn't matter what, just something. Keep doing that for a while. To make your life easier, get a book about form. Actually, really, you're going to be better off taking some classes at a community college or finding a private tutor. If you want to be a good composer you'll save yourself a lot of time if you go and take a class.
I'm not sure what you mean by 'come up with the key signature and time signature.' The way I do it -which I'm not saying is the best way or the way you should compose- I figure out what key signature I'm playing in afterwards, and the time signature is worked out that way too. However, you can also decide that you want to write a C major piece in 3/4 time and go from there. Your call.
Regards tools: Here's what I do: I have my keyboard, some staff paper I printed off the internet, a clipboard, a mechanical pencil, and a metronome. Oh, and some headphones. I'm old-school :P The advantage of this system is that it is virtually free. The disadvantage is that it takes quite a bit more time than software.
If you're going to use software -and you should get used to at least one program- you'll want to find a sequencer and some notation software. Sibelius and Finale are the two most popular composing programs, though there are others. If all goes well I should be set up with Sonar 7 tomorrow, which is also used. You'll want to have a keyboard set up so you can input your melodies without having to click through each individual note. Thing is, you're looking at about $600 in software and more in hardware. Hooray for academic discounts! Software like Sonar is cheaper. You can also look around and see if anyone can set you up with software in some sort of cheap and legal way.
I can't tell you have to avoid making your music sound like other music. That is the bane of every composer's existence. The more you compose the better you will get at composing and the more your voice will come out. The question really is, "How unique is my inner musical voice," and that's something nobody can answer. Whether you think it was nature or God or whatever, nobody will deny that people have varying levels of ability in different things. Nobody can tell if you're going to be the next Sousa.
It sounds like you're not expecting everything to fall in to place right away, which is good. Go pick up a staff paper notebook -it's better than using loose staff paper, trust me- and start making little melodies. Doesn't matter if they're any good -honestly they probably won't be all that great but every composer has to start somewhere, and practice makes perfect- you just need to write them down. Congratulations, you just wrote your first musical sketches.
Here are my last bits of advice:
-When asking for criticism of your work, be specific.
-If someone says, "Can I make a suggestion," say no. Trust me on this one. EDIT: Well, I wrote a whole bunch more than this but Yahoo cut it off. Sorry :(