All really good advice in here; although some of it may be overwhelming at first. Regardless of what virtual instrument library you choose to use, you're going to spend a decent amount of time getting it to sound right. You really need to ask yourself what the music is for.
Example: When I don't have a project to work on, I tend to write very ambient, lush and textural sounding music; I use a healthy amount of sustain and dynamic fading, reverb, room delay, etc. This works for the type of music I choose to create, as you can you can fill a lot of space with just a few instruments.
But this would not be the type of thing you'd want to do if you're working on a piece with fast movement and many notes. It'll sound muddy and indecipherable if you apply the same approach. This is why music used to be, and still is for the most part, written for the room it will be played in. Hip Hop wouldn't sound very good blasting in an ancient cathedral, just as Gregorian chanting doesn't sound best in a bathroom. If you're writing a fast moving piece, maybe staccato, or pizzicato and little to no room reverb is what will make the instruments sound best. Let your ears be your ultimate guide, but knowing what you want beforehand can save you a lot of time and stress, which will ultimately make better music.