Remember, it's okay to look like a newb, but if you blatantly disregard common RP etiquette, you will look like a jerk, not a newb. The most important rule is don't be a jackass. Respect other players, if you don't understand or are unclear about something, ask someone.
Rule #1. IC vs OOC:
A common practice for speaking In Character (IC) and out of character (OOC) is to use brackets when you are speaking OOC. Such as:
In character: Hi, how are you today?
Out of Character: ((Hi, how are you today))
Some people simply say "ooc: then whatever you want to say here", though it is more common to use brackets.
Rule #2. Grammar, Grammar Grammar:
Practice proper grammar, capitilazation and punctuation. If you make mistakes, that is fine, but avoid leet speak and use full words. For instance, don't say "Hai how R U?" Also, don't use term's like "LOL". If something is funny, emote a laugh or say ((lol)).
Rule #3. Stay inside your element:
Stick to the lore of the environment your playing in. If you are roleplaying in a fantasy setting, don't start talking about your Light Saber. Also, try to find a way to rephrase common game terms to speak in an RP manner. If you can't think of a way to do this, then use OOC comments.
Rule #4. Forceful Roleplay or Godmodding:
Do not force actions on another player. This is referred to as GodModding. For example, in an RP scene do not grab someone and throw them against a wall, if you would like to do something like that, emote "grabs
Rule #5. Did I say you could interrupt me?
Don't interrupt the RP of others. If you were to walk up in the middle of someone's conversation, in real life, and just start talking away people would consider that rude. The same goes for RP. There are more polite ways to introduce yourself to their roleplay, an acceptable method is to whisper one of the members something along the lines of ((Hello, I don't want to include myself in your roleplay without asking, would it be okay for me to engage in roleplay with this group?)). A lot of the time people are more than happy to meet and accept other roleplayers. There are times however, when it is a more private conversation and they don't want random people joining.
You can also tell if a conversation may be limited to those in it, by listening for a short time. Does it sound like something private? If so, step away and find other roleplay.
Remember, if you are allowed in the conversation do not step in and try to run it. Work with what is currently going on without taking it over and making everyone focus on you. It is okay to add occasional emotes that show off who you are or your roleplay style, but do not spam the rest of the roleplay. Perhaps you roleplay a loud and obnoxious character, this must be done carefully and appropriate as not to actually be obnoxious. If you annoy the other roleplayers, nobody will want to roleplay with you in the future, and how much fun would you have then?
Rule #6. Observing others roleplay:
This should be done in a way where you don't seem like you are hovering over them. It make others nervous, its annoying and its just not fun. This is not to say you cannot sit and observe the scene, it just means you should do so appropriately.
Standing in the middle of someones conversation and not saying anything.
Standing in the middle, or just outside, of a conversation and emoting random things..like "coughs" "combs their hair" without ever saying anything or reaching out to the other roleplayers.
Walking back and forth past them, or through them.
Jumping up and down around them, running around them in circles.
Sit off to the side, perhaps a bench if there is one. If not, find a window to look through (if your inside) or window shop (if your outside). Also casually emote something every so often.
Overall, remember that even if you follow these rules some people may still not RP with you, or even be rude. This is few and far between, but not everyone who roleplays is encouraging and accepting. There are a few elitist jerks. If you run into these types, make a note of their name and continue on.