The challenge that’s unique to acoustic composition is learning to understand and appreciate each instrument you’re writing for. Just like how each person you meet is a unique individual with varying strengths and weaknesses, every instrument in an ensemble is a world unto itself. Some composers spend their entire career only composing for a few select instruments, while others have a diverse catalogue. It’s up to you as a composer to decide what you want to compose for.
Research the instrumentation of common small chamber ensembles. Choose one that you would like to compose an original piece for. Sketch 3 melodic ideas that are idiomatically correct for your instrumentation.