Usually the first thing newcomers ask about forum RP is "How do you know who's turn it is?". Well it's simple; most threads use a 'round-robin' rotation basically meaning each person takes it in turns. One person starts the thread and the people participating reply in any order, then when everyone's posted it goes back to the first person and follows through in whatever order people first posted in. In standard threads this is the way turns are decided. Now sometimes one person in your thread might say "Hey, it's okay to skip me, I don't have much to write here", and that's perfectly fine. If anything, it's better than them churning out one sentence just to take up their turn. Don't assume you can skip someone's turn though, atleast ask through message or other means if it's okay.
Alot of forums, including this one, have a 'mention' feature which lets you notify a person (or multiple people) when you've posted to them. To use this feature, simply put an @ symbol infront of and "speech marks" around their name (for example @"Name"), maybe at the end of your post (leave a line/space between your post and the mention). This will give them a notification in the top right-hand corner of the site, allowing them to simply click and go to your post. It's best to tag the person or people you're RPing with in every post, in case they don't follow the thread. In bigger threads where you aren't in the same location as everyone, or you're only fighting/interacting with one or two people, only tag the people close to and relevant to your character's actions.
Detail
"How much do I write? Do I have to write a novel every post?" - Other frequent questions from people used to a shorter style of writing. Some forums have a minimum amount you must write in a post, but most (including this one) let you write as little or as much as you like. It depends on who you're in a thread with. In any case try not to write just one or two sentences, you want to detail what your character is doing and saying so other writers can play off that. Don't give too little detail and leave it to someone else to create the world and carry the creativity. However, at the same time don't swamp your writing partner with a novel of a post, especially if they don't achieve the same standard of writing. Achieve a balance between each other's writing and all should be fine.
Combat
Various games and sites have different ways of roleplaying combat. MMORPG roleplayers sometimes use dice rolls to decide what attacks do or do not hit, whilst other chat-based games rely on speed typing or sometimes consent of harm. On most forum sites, including here, we use the consent method. This means you can not kill or seriously harm another writer's character without their consent. In combat, your attacks outcomes remain ambigious; the most common word used should be 'attempt'. Below are some examples of good and bad combat roleplays:
Bad
Dan unleashed a barrage of blue fire at Ven, burning him alive in a torrent of flames until nothing but ash remained.
Good
Dan unleashed a barrage of blue fire at Ven, attempting to burn him alive in the hope that nothing but ash would remain.
The Bad example leaves Ven with no choice but to die, whereas the Good example allows Ven to choose whether to block or take the hit in his next post. One thing to remember though is not to block or dodge absolutely everything. The key to a good posting fight is giving and taking
balanced hits. For example, if Ven lasts a whole fight without once taking a hit from Dan, despite Dan attempting numerous times, then he is not taking balanced hits. A better outcome would be if on Dan's third attempt, Ven had taken a fireball to his left side, burning his arm and leaving a nasty mark. This way you aren't godmoding your way to victory, and you can remember your scars for your next encounter...
"But how is a winner decided?" - Before a fight the writers can either agree the outcome beforehand OOC, which usually happens if a specific storyline is aiming to be followed, or they can allow the fight to play out and see who wins in the end. Smart thinking and descriptive moves can help win a fight. If your opponent uses a clever tactic, something in their environment or something you weren't expecting, then take a balanced hit; they probably deserve it. Don't assume your character's power will win you the fight; it's all down to the writers.
If you're fighting NPCs though, then you don't have to skirt around hitting them. One writer in your thread may be acting as the Storyteller or 'DM', the ones who're leading the story and writing in the enemies for you to fight. Most of the time your standard grunts, soldiers and trash can be killed in one post. Powerful characters might cleave their way through these guys like it was nobodies business. However don't assume all NPCs can be one-shot. If a writer has put effort into describing the skyscraper-sized behemoth of a demon, don't just cut it's head off straight away because it's an NPC.
That about does it for the Basics of Forum RP, you should be able to apply this tips to any RP Forum you come across.
For more specific explanations and tips for this site, check out the other guides.