Tag style:

I have done a post and reply, so hopefully all is covered. Notes in gold explain. 

With this style you have full control of your character and what he/she says or does. However, you also have limited control over other people’s characters. While you can try and steer them into saying or doing what you want them to, they decided their characters actions. This style can be written in present or past tense in first person, third person or even in the illusive second person. However, the third person past tense is by far the most common

the reply:

Things to remember when posting:

• Start off each post with an objective: it will help make your post mean something in the ultimate story. No one is going to want to read loads of stuff that has nothing to do with the plot and doesn't get anywhere, in the above post the objective accomplished is that Spivik learns about Betty's past.
• Try to involve as many PCs as possible: this gets your character into the social swing of things
• Don't rely on others: if you want your character to become involved in the story you can't rely on others to get them involved, YOU have to get them involved because they're YOUR character.
• Always think of others: don't write for someone else's character unless you're sure how that character will react. respect others and they will respect you.
• Keep on friendly terms with the captain: this doesn't mean suck up, it means don't pick fights unnecessarily or argue with his or her decisions. Remember what the name 'captain' implies. IC, OOC and NM!
• USE A SPELL CHECKER or dictionary: Nothing's more embarrassing then a typo, well maybe some things, but not many.
• Use a thesaurus: the more interesting your posts are - the more people will want to read them
• Develop your character: drop little insights into your character here and there so that you keep other players interested in him/her, however, make sure that they don't come out of the blue and have nothing to do with the story.
• Don't play hero: Nobody likes a winner, rather a slow beginner. Actually, the more faults your character has the more people seem to like him/her... just don't get too melodramatic!
• Be tactful: if you're nice others are more likely to like you and your character.
• Unless you know everyone in the group has HTML supported e-mail, use plain text for your posts: this is just so that it's readable by everyone. You don't want to send a great post and then have no-one able to read it!

OK. Now what?

Subscribe to a group (see: what is a RPG), find out your group's address and particular posting style, talk (write) to your captain and then start posting!

NOTES:

• Don't get a fright when your post comes back to you, unless it says 'mail delivery system failed' it's nothing to worry about. Many groups work on posting lists such as Yahoo groups or Topica. Once you subscribe your name goes on a list and the posting list will send all mail sent to the group's address to everyone - including you.
• In order to join to the group you need to subscribe to the mailing list. I don't know about others but if the group is on Yahoo then you need a yahoo account to do this. Here's how you get one. It's a bit of reading so you might want to print it - that's why it's in black and white. I'm sure the others work the same way.
• Read descriptions of the group before you join it. If you find out that you don't like it later you can't exactly say 'So long, I hate your group!', well you can, but you might never end up being able to RPG again. The RPG world is very small.