I've written a lot in my time, and I've learned a few do's and don'ts concerning the art of writing. If you're unsure of your writing skills or simply starting out, I'd recommend that you read this tutorial thoroughly.
Firstly, everybody has a time to write. Not even the greatest authors in the world can pick up a pen (or sit at a keyboard) and write whenever they want to. Sometimes you'll be in a mood to write, and other times you won't be... trying to force yourself to write when you don't want to often has bad results.
When you're writing a story, you should at least take the time to capitalise the start of a sentance, people's names, place names, and so on. Just press the shift key, press a letter key, and let go of the shift key. It's not that hard, so it's amazing why people insist on writing with no capitalisation not just on instant messengers, but in stories and fanfics on the internet. Remember that the letter 'i' on its own must always be capitalised ("What did I tell you?").
When you're writing a lengthy piece, you should break your text up into paragraphs and indent them with a press of the Tab key. It makes the paragraphs stand out and makes your story or whatever easier to read. Leaving a space between paragraphs is optional, but it helps divide up paragraphs even more.
Punctuation is another thing that suffers when people write on the 'net... people often just miss off periods, apostrophes, commas, and quotation marks all over the place. This is all right if you're in an instant message conversation with someone, but if you're writing a story or essay or something it's important to remember to break up long sentances with commas and put a period at the end of every sentance.
Remember to spell check your work - Microsoft Word may be as thick as a log when it comes to grammar, but it knows when you've spelt a word wrong, so pick the correct spelling and move on. If you make few or almost no errors when you write something it looks a lot more professional then something with a lot of errors.
If you need to use transition (switching views, for example), you should leave a gap between paragraphs larger than the normal gap. If you don't leave any gaps between paragraphs then you only need to leave a single line, but if you leave a line between every paragraph then be sure to leave three lines when you make a transition.
Finally, don't use multiple exclamation marks or question marks at the end of an exclamation or question ("Dude!!!!!!", or "What?????")... it just looks wrong. If you want to make something sound surprised and shocked, you can always use two question marks and an exclamation mark ("You what?!?"), but it's not recommended. And the "!!!1111oneoneone" joke has been done to death. Don't use it, even in jest.