On the top of this page go to the "Editors Depot" and on the right theres a "Tutorials" tab. Select that and then choose OFP as the game. On page 3 is a "Briefing.html file tutorial" by Snypir. Download it and read it.
That document covers how to use an html file to write a briefing. Personally I use Vista an half the time the way it saves the html files I never get the briefing to show up, and Ive never managed to get a picture to show up so I never submit any missions here (read the minimun requirements for submission) But then again I'm about as OFP technically inclined as a fruit bat.
Read through as many beginner tutorials as you can, they are an incredible help.