If you're a beginner and start to use Sibelius, then you have as much chance as a snowball in hell.

Sibelius is primarily for writing out your music in note form - sheet music. And it does it abnormally well.

If you just want to use it to create MIDI's then it is a real waste of space and time. Instead use what sciencekid has recommended. It's a good program for just churning out single instrument MIDIs. There are probably other programs that can make many instrument MIDIs, and I believe you might be able to do in Anvil, but oh well...

So, recap:

Beginners (who know nothing or very little about the theory of music and what not) - Use Anvil
Musicians (who know some of the theory to all of it) - Use Sibelius