It was easier for many composers a few hundred years ago. For example, Haydn had his own private orchestra! No wonder he wrote so many symphonies.
I can make up music in my head, and hum it etc., but I often find it hard to write down. Sometimes I might improvise something on the piano, decide that I like the sound of it, and then realise that i've forgotten it. It happens quite a bit! That's why I've had to start recording myself when I'm composing.
In my opinion, composing for string quartet is more difficult than for full orchestra. With an orchestra, most of the instruments will be playing the same melody anyways, but with a string quartet, you have to imagine 4 different melody lines at the same time. That's why programs like Finale and Sibelius can be useful.
It's my dream to compose piano concertos, string quartets and even a symphony, but I'm only a teenager now, so I'll stick to solo piano waltzes, preludes and maybe the odd sonata.