Don't know what to write? Follow this to help you get started.
Describe the weather. The scenery. What do you see when you look around? Describe what you smell, feel, taste. Think. Everyone has thoughts. What are yours? If this isn't your first post, react to everything the other character does on the previous post. Here's an example of a short starting post, the very least of what you should write.
A single drop of rain fell from the sky, hitting Byakuya's head. He looked up, startled, and saw that the sky had turned a dark gray. Byakuya sighed, shaking his head. He didn't like rain. With rain came mud, and mud meant he was likely to get his clothes dirty. Hopefully it stops soon, before the sixth division goes to the human world, he thought, walking quickly toward his manor.
A starting post should be at least two paragraphs long, but that shows some things you can say in your post.