Sometimes when joining a site like this for the first time can be difficult to adapt to. My earliest posts struggled to go over a paragraph and had limited to no punctuation whatsoever. This is a helpful guide for roleplay beginners that can be used to really enhance your posts.
#1: READ; If you aren't the first person in the post, (Say someone else started it) Read every section of their post, from top to bottom, take in what they said, if they described clothes, the weather, where they were before, make sure you know the back story before you post.
#2: I know this doesn't sound like it will help your posting.. but Punctuation and Capitalization.. It sounds stupid, I know, but when someone can actually read the sentences put down in the thread, it not only makes it easier for the other person to respond, but it puts the ideas together for you, and makes it easier for YOU to see where the sentences are split and where they begin again.
#3: Description, Description, Description; Long posts are what people like to read, they like to know everything about your character, including:
-Where they were before the post started (If it's the first post or your first reply to a topic)
-what they are wearing, I like to know everything from hats to shoes. Even if it doesn't seem important, put it in! Not only does it raise word count, but it gives a lot more description (Again, clothes are for either beginning posts for the topic, or first replies to the topic, clothes don't need to be mentioned every post, unless there is something making your clothes stand out, such as a wind, or it's sunny and you have a bright shirt on and it hurts your characters eyes)
- Weather: (Again first or first reply to topic) Is it sunny? Raining? Did the thunder scare your character? Why did it scare them? There are so many questions you can ask yourself not only about your character, but what is going on and why.
-Remember, everyone thinks, and remembers. If something someone just said made your character remember something else, say it! Sometimes if it sounds better in first person as a thought, put it in italics, so everyone knows, "This is what I'm thinking."
-If there is a fight: Remember to keep it real. Everyone get hit, and it always depends on the situation. Is it a muggle area? Do you even know if the person you are talking to is a witch or wizard? Obstacles in your way, say you're in a fight and your character trips and falls, and when you trip, you get hurt! Blood on your hands and knees, imagine if you were to fall on concrete, it would hurt too, how does the pain feel? Like it's on fire? Like needles pricking your skin? If you got hit by a Cruciatus curse, what does it feel like? Like every atom in your body was set on fire? like thousands of pins are stabbing you? Everyone feels it differently.
#3: Paragraphs; Every person has a train of thought. And this is no different. When something feels like it has been adequately described, separate it in a paragraph! It'll not only make the post feel longer, but look longer and feel more organized.
#4: Spelling; Most programs have an automatic spell check, use it to your advantage! Sometimes I don't know that a word is spelled wrong until I see it underlined in red. I currently use Internet Explorer for Windows 8.1. Google Chrome I believe has the feature too!
This is everything I could think of and I really hope it helps!