Type your text in the box and press save- you may have to press the Reload button on your browser to see your edits.
WikiWikiWeb s use a markup language to format the text- don't bother typing in HTML, it won't work. Check out WikiFormatting for more info on formatting rules.
OneMinuteWiki has some good tips on getting the most out of this Wiki.
It's usually considered bad form to delete things from the Wiki- if a page is getting too cluttered, make a NewPage and link to it.