HowToUseAWikiRecentChanges is a really good place to start.
Editing
If you click on the Preferences link, you can assign yourself a name which will appear there on any changes you make.
Some pages are better treated as a living document rather than a conversation; this page is one such. The aim is to produce a list of hints for a new user, or reference material for an experienced one. So a remark like "...but I don't know how to (whatever)" is better answered by replacement rather than by adding an answer and leaving the question untouched. WikisAreNotUsenet.
Look at context when editing. It's all too easy to change half a paragraph and leave the other half looking stupid or irrelevant.
Formatting
(To see how effects on this page were generated use the Edit text of this page link at the bottom and take a look at the source. If you want to experiment, you can use MessAround.)
Links. Page names using StudlyCaps are automatically recognized as such (sometimes these are called WikiNames). If you want to have page names in other formats, like Toll, then you have to put them in double square brackets when referring to them, for example [[Toll]].
Square brackets produce a link to an arbitrary URL, e.g. [http://www.chiark.greenend.org.uk/ Chiark] produces [Chiark]. If the software recognizes the extension on the end of the URL as an image format (currently: gif, jpg, png, bmp, jpeg) then you'll get an inline image instead of just a link.
Text Style. Use ''two apostrophes'' around things to get this style (HTML "em" element) and '''three apostrophes''' to get this one (HTML "strong" element).
Lists. You can have bulleted or numbered lists by putting a star or hash right at the start of the line. Suppress the special meaning of the lead character with <nowiki> tags.
(Don't put a space before the * - that means something different; see below.)
Preformatted text. Leading spaces allow you to use preformatted, monospaced text, like this:
o o
|
\_/
Also, you can put small spans of text inside a <nowiki>...</nowiki> psuedo-element, to suppress special meanings otherwise assigned to the characters.
Indentation. One or more colons at the start of the line indent the paragraph by one or more steps, like this:
Rules. Multiple (at least 4) dashes or equals signs on the beginning of a line produce a horizontal rule. Some browsers may display the two kinds slightly differently.
Other
See also HowNotToUseAWiki.