During the process of writing your roleplay post, you will have to think about two different parts: description, and dialogue. It's just like when you write a story, except you have to wait for your partner's part for the story to progress. Basically, your post will contain your part of the roleplay (your character's actions), and the next post will contain your partner's part (his character's actions).
Here's an example of what it could look like:
Post 1 (your post):
John was walking around the Palace, looking for his friend.
"Jake, where are you?", he called.
Post 2 (your partner's post):
Jake came out of the kitchens just in time to hear John calling.
"I'm here! I was just getting a snack..."
You obviously have to write more, but basically, that's it! If you're having trouble figuring out what to write, just try to be in character. Write how your character reacts, how he feels, describe what's around him, his actions, etc. You can also look at other topics to get inspiration. When you think about it, 150 words is not a lot, and you should be able to cover it with a bit of practice. If you're still not sure you understand everything, you can contact the GCat. Otherwise, you can start reading the documentation. Good luck, and have fun!