I'm actually going to have to disagree a bit with the notion some have that a better sample library won't do you any good unless your orchestration skills are better first.
It's hard to practice that unless you actually have a sample library that has a lot of articulations and is capable of making something realistic. For example, you will never be able to create a truly realistic sounding, expressive violin melody without legato, portamento, marcato, etc. Players generally don't play with just one articulation (a simple sustain in most crap libraries) throughout a phrase and playing with the envelope usually doesn't produce as satisfying results.
If you know this, but your library only has something like sustains and staccatos, you will likely avoid writing melodies that use shorter notes because you'll notice the staccatos are too short and sound unnatural in your phrase - you need a detache, marcato or some other sort of short sample with a longer decay/release instead.
I've never understood the idea that lots of people have that you should start with something low-end and garbage and then move your way up when a smarter financial decision is to buy something really good and improve yourself so that when you are good enough, you won't need to spend any more money.