Treat it as if you're writing a story but live. There are no strict rules, but it is good to do it in this sort of style: He moved slowly towards the table, drawing out a hand and opening his mouth to speak, "What is this?"
Try to match the amount of material written by the other player(s).
if they type a paragraph or two, then try to match approximately that amount. Nothing kills a game faster than typing full paragraphs of material and getting only single sentences in return. At the same time, if players want to take their time and fill out a proper dialogue, don't take control of the situation and change the subject on them. Pace your actions and the flow of your story as well. Dialogue is usually spoken in single lines and tiny actions. Descriptive actions and events can get kind of wordy sometimes so keep things creative but don't go overboard.
Don't take control of other player(s) characters unless they're okay with it.
Don't put words into their mouths either.
Don't plan too far ahead and/or take too much control of the game/story.
You're playing a creative game with other creative people. Collaborate! Cooperate! Let the other player(s) have some fun and they will hopefully give you some creative control too. The game doesn't usually play like Dungeons and Dragon's wherein one player has more control than the others. Don't do things or change situations in a manner you wouldn't want others to. RP is a game of give and take.