Contributing to the wiki
To edit the wiki, you just need to register an account. With the exception of the Main Page, you can just click on "Actions" then "Edit".
To discuss the wiki's content or make suggestions, you can either use the relevant page's talk page (click "Discussion") or the Community portal page.
To sign your name on a talk page, just type the tilde character four times (~~~~) and it will be converted into your name and time when you save the page.
IFWiki in particular
The expected content of IFWiki is discussed on the various pages within Category:Style guides.
Wikis in general
The software behind IFWiki is called MediaWiki. You can edit pages with a combination of wikitext (a markup language) and HTML. Two pages on IFWiki (which are largely themselves copied and pasted from the MediaWiki website) discuss this markup: Help:Editing and Help:Table.
You may also find the The "Help:Contents" page on MediaWiki's website useful.