You'll notice a lot of edit buttons on pages with lots of headings, that allows you to edit just a section of a page. Note than when you edit something, the page locks so others can't edit it at the same time.
To edit a whole page at once, use the top tool icon (the pencil) on the far right sidebar with all the icons. If on a mobile device, it shows instead as a dropdown box of tools to the right of the search box (edit this page).
You can use the toolbar to insert formatting or type the wiki code in directly (click on the link “syntax” when editing to see a list of them)
It's difficult to keep secrets from other players in your campaign, so please see hiding text.
When you edit a page, you get a toolbar at the the top of the edit box with various formatting icons. These will generally insert the correct syntax wiki code (the symbols that tell the wiki how to format something) at the cursor location in the edit box.
The last symbol (pen writing the end of a paragraph) will insert your signature. This is just your name, an email link and a timestamp to let folk know you are taking responsibility for the content you just added this too. It's not mandatory of anything, but is a handy tool of you want quick feedback on something.