Editing a page is as simple as clicking on the "Edit" icon at the top of the screen or the edit link in the page footer.
There are many possible options for markup. For your convenience, there is a formatting toolbar, which works very much like a toolbar in a word processor. By highlighting text and clicking one of the buttons, such as Bold, the text will be surrounded by the appropriate markup to make it appear bold.
Always make sure you "Preview" before you "Save Changes", otherwise you might end up editing a page several times.
The following pages describe the elements (wiki markup) you can use to get special formatting effects:
Help on Formatting - general text formatting rules
Help on Linking - external hyperlinks and inline images
Help on Headlines - section headings
Help on Lists - lists and indented sections
Help on Tables - tables
Help with Images - images
Help with Macros - macros are magic things [[LikeThis]]
To experiment with wiki markup, go to the Sandbox and then click on the Edit tab on the top of the page. Use your browser's "open a new window with this link" feature on the word "Wiki Sand Box", so you can keep the help pages open side-by-side to the editing window.
For a list of other Help topics, see the Help Documentation.