The Briefing File
Once a player has selected your mission and clicked 'Fly', they are taken to a briefing page. This is simply an HTML file, with a description of what the mission is about. All of the default briefings follow the same style and, while you're free to come up with your own unique style, keeping to the same one maintains the 'look and feel' of FSX and makes your mission fit well with the rest.
You can copy any of the existing briefing files, found in the various folders under the Missions folder, and edit those. Likewise, any HTML editor can be used to edit them. Both MS Word and the free OpenOffice can visually edit HTML, although both make lots of changes without your permission and you can end up with extremely large HTML files. Alternatively, any text editor such as Notepad or Notepad2 will let you edit the HTML directly.
Once your briefing file is created, you need to set your ScenarioMetadata object to point at it so that FSX knows what file to load.