I know from personal experience just how daunting it can be when you first enter the RPG forum and try and work out what's going on, where you're meant to post, characters, stories, rules, so on and so forth.
So here's a little bit of advice from the moderator of the RPG forum to help out. Total newbies to RPing should read this if you need to, and also anyone RPing in HE Forums for the first time might find this helpful.

What is an RPG?

An RPG (Role Playing Game) is a text-based game that we play by constructing a story through individual posts.
Players must pick an RPG they wish to join, create one or more character (see below for more info) and then take it in turns with the other people in the RPG to post the actions your character carries out, moving around the area, interacting with other characters and the environment and so on.
You might find it useful to read a few pages of an existing RPG to get you in the mood of things.

Making and posting a character

Obviously the characters you make are going to be very important for you and for the RPG you join. It annoys people a lot and can mess up an RPG story if you create a character and then kill it off quickly or just leave it standing without ever doing anything with it if you change your mind about the character you want. Also making lots of characters very quickly is a bad idea, especially if you're RPing for the first time. My recommendation would be a maximum of three characters or less when you join. Wait until you're settled in to start making more. Think about your character carefully before you make it so you know you'll be happy once you start.
To make a character, fill in a character profile. Most RPGs should give the frame for the profile in the first post. Alternatively use one like this:

Name: (Your character's name)
Species: (Human, monster, creature and so on)
Age: (How old your character is, some people do it in comparison to a human. You don't have to be precise)
Gender: (Male or female, or something else)
Appearance: (What the character looks like)
Personality: (likes, dislikes, quirks, what makes the character interesting)
Equipment: (What the character carries)
History: (Your character's past)

You don't have to include all of these, but the more there is, the more we know of the character. Equipment and History are probably the least important two, as these can be developed during the RPG. Also, you don't have to confine yourself just to the things I've listed. If you have any other ideas you can add them.
The profile can be short, but it's good to take the opportunity to describe your character well here. It might help to read the profiles of other existing characters to give you an idea.
If you do want to get rid of one of your characters after a while, make sure it fits in with the story and make a good reason for him/her to leave. Sometimes there are times when it makes sense for a character to leave the RPG.

RPing

The first thing you should do is check the RPG rules thread. Then check the first post of the RPG you want to join as this may include additional rules and information. If you're joining a longer RPG it’s a good idea to read the past two or three pages so you can get the idea of what is going on. Make sure you understand the story of the RPG you are joining.
Once you start, be aware of others and stay open to the advice of other players. One of the most important things to do is be patient. Many members can't get on every day so don't expect a reply to your posts straight away. If you start to find almost every second post is one of yours, slow down a little. Also avoid having a series of short posts between yourself and another member when you can include a longer, more detailed post. It's easier for other players and makes the RPG appear friendlier for players who join later. More information can be found in the rules.
Remember, RPing is about interacting with other people and their characters. Avoid making a string of posts where you don't interact with other characters. There might be times when your posts are based around your characters alone, but try to interact with others, or give others the opportunity to interact with you.

How do I set out my posts?

If you want to speak with the other players begin with
'OOC:' (out of character)
Once you start with the game, type
'IC:' (in character)
There are two main ways of laying out a post. First is the script format which goes something like this.
Character: "Character says something"
Character: (Character does something)
Character's thoughts: 'Character thinks something'

So for example:
Adam Monroe: (Raises sword and begins killing parasite) "This is what you get for slacking off, slave!"
Adam's thoughts: 'I love killing Parasites!'

The second way of laying out a post is the story format where you lay out the post like you would a story. Example:
Adam Monroe raised his sword and began killing the manipulated parasite. "This is what you get for slacking off, slave!" He continued to kill it.
'Interesting, I love killing Parasites,' He thought insanely.

You can experiment with them, most find script better at first. Some people use a combination of them both. By reading existing posts you can get a better idea of how RPing works.