To edit a page, log in and then click on the "edit" tab at the top of the page. A box will then appear into which you put the contents of the page. Rather than enter raw HTML, you enter a simplified version of HTML called wiki syntax. The following webpages explain futher:
The best way to learn the wiki syntax is to follow by example.
The formatting of the entries in the wiki is a compromise between making each entry readable and making it easy to enter in the wiki syntax. There are no formal guidelines; follow the previously existing formatting.