Point one: Kanji
Do not greet everyone with "今日は".
Kanji is cool. We all want to use kanji because it looks more smart. IME is great, because you don't have to know how to write the kanji to use it. The problem is, there are some words that simply don't use kanji. One of them is こんにちは. Yes, an obscure reading of 今日 is "こんにち", but 99.9% of Japanese readers will see 今日 and think きょう. I can't count the times I've stumbled upon "今日は！" and thought "Today is... Today is what? What is this person saying about today? I'm so confused..."
The best rule is, only use kanji you know how to write. If you can't follow that, the next best rule is only use kanji you know how to read. If you can't do that, then you should at least only use kanji in a way you've seen it used before (by native Japanese writers). Since こんにちは is never written as "今日は", you should never make the mistake of misusing the kanji like this.
Other words that usually don't use kanji:
すごい
どこ
すぐ
なる
する
いる
ある
-みたい (looks like)
-てほしい
Point two: Politeness
In Japanese, an exremely important thing to learn is politeness levels. Many people think if they don't go to Japan, they don't need to worry about politeness. However, it's such an important part of the language and culture that you would be doing yourself a great injustice by overlooking it. Your friends probably wouldn't notice the difference, but when communicating with a Japanese person, your politeness levels will be very evident and can affect the relationship as such. I'm not even Japanese, but whenever I see the wrong politeness levels used it definately affects my feelings (i.e. when someone I thought was a close friend uses distanced speech, or someone I don't know at all speaks very informally, I feel a bit put off).
A quick overview:
-- Friends: In e-mails and MSN conversations with friends, informal/plain form is fine. Feel free to use any slang or contractions you want.
あのさぁ、昨日けんちゃんが俺んちに来たんだよ。
-- Acquaintances: In e-mails and MSN conversations with people you don't really know (i.e. penpals, people you met on this site, etc.) you need to speak politely. Even if you feel like you have a very friendly relationship, this politeness is very important. Keigo (very polite speech) is usually only learned by advanced students, but that fact is known by just about everyone
Just using the polite form and choosing humble words will go a long way.こんにちは。最近元気でしたか？今日学校が休みでしたから、私は友達と遊びました。
-- Public: When posting messages on this forum, you are addressing the public (i.e. the whole world). We are all friends here, and the highest form of keigo you can throw together is very much an overkill, but speaking informally, like you would to a close friend, is inappropriate. As I sort of mentioned before, I have often come accross posts by people I haven't heard from before, directed at everyone, using informal speech. Reading these makes me think "I don't know you, why are you talking down to me? You must not have much experience speaking Japanese." Aside from making people enjoy your writings more, speaking more politely will make your Japanese abilities appear much more skillful. The writing style for this is very similar to #2.
When addressing the public, do not refer to yourself as 俺(ore).
Posting a message on the forum directed at someone you know well and are on an "informal" basis with requires a combination of the forms above. You are still communicating in a public domain, which means informal speech is inappropriate. Formal speech (like #2) is fine, but if you want to convey informality, do so with caution. It is possible to mix forms, but learning how to do so takes some experience. Adding a little bit of formality to the informal makes it good enough for the public, but soft enough for your friend.
私もそう思います！とても面白い。Ｔｏｎｙも説明してくれない？楽しみに待ってます。
For a good example of how to address the public, pay attention to the way Pipin and Coco address various people.
(PipinさんとCocoさん：皆はＰさんとＣさんのポストに正しい丁寧さを習えると思います。恥ずかしかったら、ごめんなさい！今からも、普通に書いてください)
