To edit a Wiki page, you first need to log in. See Signing Up And Logging In for more information.
You should then see a tab that says "Edit", either just below the page title or at the very bottom of the page, depending on what theme you are using. Click that to enter the editing interface.
You should be able to mostly just type whatever you want, but if you want to do anything complicated, either click the tab in the editing screen that says "Wiki Quick Help", or read Wiki Page Editor for more information. The Wiki Quick Help is often a bit more up to date, however. Note that paragraphs are seperated by blank lines, so you can hit return (once) anywhere you like.
The "minor" button means that people subscribed to WikiChanges will not get mail about your edits.
As a stylistic note, since this Wiki is meant for content related to Lojban language, and Lojban language makes very little use of capital letters, and in fact tends to look ugly if you capitalize it, we require the use of double parentheses, (( )), around links, rather than WikiWords.
To create a new page, just link to it from another page.
If you do a lot of editing, you might find the quick edit module convenient; this allows you to create a page without having to link it from another page. Go to MyTiki -> Modules to set it up.
If you do a lot of editing, you may want to sign up for the WikiDiscuss list, which causes anything anyone says in a "discuss" tab to be mailed to you. See Tiki Features And Information.