Think about what you like and what you want for the room. Do you want to relax, sleep, work, entertain? in the room? That will determine whether how you decorate. Then, think about the colors you like - earth tones, jewel tones, pastels, bright colors?
Now, you can think about the furniture you like - do you like classical furniture, overstuffed, etc.
All these things help you decide how to decorate your room.