You can't actually attach a file to a post, unfortunately (as you might with an email). You'll need to upload your document to a websever and post a link on the forum.
Uploading is the tricky bit and I can't give you much specific help, sadly. I used the free webspace provided by my ISP and they gave instructions for how to do it. Yours should too. If you don't have webspace from your ISP there are websites (such as immageshack) who will host your documents for you. The ones I know seem to be for pictures only but maybe some kind soul can suggest a good one. Again, whereever you use should give specific instructions on getting your file online.
You will need an ftp client program to actually get your file onto the host server. I used SmartFTP.
Once that's over you should have a shiny new web address for your file and just need to post the link. You can just put the web address in the post, or use the following construction:
(url=http://your.web.address)What gets shown on the post(/url)
Only use square brackets [ ], not round ones. (Hey, it's tricky finding a way to show the full construction in a post.)
(Take a look at the "FG's Song Review" post to see this kind of link in action.)
"If there's anything worse than being sacrificed, it's being sacrificed incompetantly."
That said, if you want your story to be read, you really should take the time to copy it into the post proper and reformat it as necessary. An external link to a word file is "extra work," and will cost you readers.
Boo - I think Leif's right 
It's simple, open your word document, highlight say, a chapter's worth, copy it to clipboard. Open your web browser and either make a new topic or if you have one already open, click on reply and then paste into the post box, and hit continue.
Done. If you keep in mind that you want to post it on a message board and you're using italics and such, just type the ubb code right into the text so you don't have to tinker with it when you come to post.
Simple as that.
Don't ask me about joining Honourable United, I'm lazy. Ask Captain Cathode.
http://www.onthejazz.co.uk/honu/
"If I had a punch, I would so hit that guy." - Millenium (because drinking nail varnish remover is for real men)
I am rubbish at the whole adding things to forums thingies and, well you all seem to be knowledgeable people - so who can tell me (in short, easy-to-understand words/sentences) how I post something I have written in Word?
I would just copy it over but there's some story-specific formatting that I don't want to have to redo here in painstaking monotony.
Ta