Step One - Consider your character.
How will your character interact with other users' characters? Are they rude, icy, obnoxious? Or are they docile and kind hearted? In the middle? Are they Council members? Are they Optivus? Are they neutral?
Knowing your character well is a vital aspect of RPing. How they act can be what singles out your character, makes it unique to you. You can't make them get along with everyone if they are meant to be a loner. That's not your character then?
Step Two - Consider the other user(s) character(s).
Will the characters get along? Do they have the same interests? Are their personalities different? You need tothink about whether your character is compatible. Although you find this out in RP, it's always good to know how your character will act around other characters.
Step Three - Where is your character?
Knowing the location can also help determine your character's interactions. Is it the comforts of home, the thrilling darkness of a thin alley, or the calming aura of a quiet forest?
It can help you truly describe what your character is seeing. Sometimes what they see can affect them. If a teenager sees a shadowy figure, will they not run from it?
It also helps what you will and won't find. You aren't going to find armour on a forest floor.
Step Four - Speech.
Always use speech marks or inverted commas (" ") or (' ') to show speaking! It helps the other user identify what is spoken. It also helps to use italics for thoughts. Does your character speak Italian? If so, do something, like an OOC note if things are spoken in French etc, like a translation. Consider what sort of wording your character would use.
Step Five - Don't Godmod, Metagame.
Godmodding is where you have your character do something that affects the other character without giving the other user control. Here's a example of godmodded roleplay:
Person A: (punches person B until they bleed)
Person B: (cuts Person A)
Person A: (strangles person B)
That is unfair. This type of RPing faux pas is totally unacceptable. Also, do NOT make your character dodge every blow. DON'T kill without the user's permission.
Here is an example of a FAIR roleplay.
Person A: (tries to punch Person B)
Person B: (is hit first, but dodges second blow and tries to cut person A)
Person A: (is given a minor wound and falls slightly.)
THAT'S BETTER. The invincibility of the characters is gone.
Step Six - Put yourself into it!