So, you're new to roleplaying, or perhaps you just want to see what I have to say. Well, nothing wrong with that. I'm just going to throw this topic up in an attempt to help you better your roleplaying skills and to possibly gather other user's opinions and ideas on good roleplaying etiquette and the like. So, without further adieu, let's start.
1. Double check your post for any grammatical errors or typos. Simple mistakes like a typo can drastically ruin your post and what you're trying to convey. Sometimes it makes what you attempted to type out nonsensical and it takes your roleplay partners out of the moment when they have to try and decipher what you meant.
2. Add supporting detail. All dialogue and a laundry list of what your character(s) is/are doing is boring and very dry. If you add in detail, describing what your character feels, how their body reacts to something, or what they're thinking it can add to the effect and lengthen at your post. You can also set up the environment by describing things around your character(s) as they go about their actions and dialogue.
3. Do not ignore another person's character or contributions to the roleplay. Whether or not you like that person or their character does not matter. It is rude and bad form to flat out ignore any one person in an roleplay if the other's character is making attempts to interact with yours.
4. Do not just piddle around. Roleplays are typically at their best when there is a storyline being followed. While this isn't necessary at least try to contribute to something bigger. Occasionally it's alright to slow the roleplay so that things like love interests, fluff, etc. etc. can progress.
5. Talk to your roleplay partners(s) in PMs or chats to determine future storyline. If you have an idea for a scenario you want to happen talk it over with your partner or partners and see what they think. When everyone in the roleplay knows what to expect it's easy to coordinate up to what you want the roleplay's overarching story to be and it generally makes it more fun for all those involved.
6. Do not godmod. This means you do not control other player's characters, make your own super-powered with abilities/skills or allow your character to wiggle out of each and every situation with no effort involved. Just like in real life you can only control yourself, not others. If you want something to happen then talk it over with your partner(s) outside of the roleplay and compromise.
7. Make quality posts. Try to make your posts of decent length and to describe what your characters are thinking, feeling or doing. It helps the other roleplayers get a handle of what you want to convey, it also shows that you care about the roleplay. Typically I try to make a minimum of four paragraphs per character in my posts, to make the post have something that my partners can reply to easily enough.
8. Don't use images, emotes or videos to convey what you want. This is lazy and uncreative. Find a way to adequately describe what you want and need to in your posts without resorting to such methods.
9. Do not just skim over other people's posts. This is rude. This is how you miss important information. Actively pay attention to what the other person as posted so that you know what they are trying to convey and head towards. It is not all about YOU, after all. Roleplaying is a joint effort usually striving towards an end goal. Help each other get to what the other wants by compromise and paying attention.