You must be logged in to make edits. The login is not connected to the Custom Gallery or the Message Board. You can register an account here.
Before you add or edit anything, you should try out editing in the Sandbox. Here you make changes without messing anything up.
Try to make your edits and new entries match other entries in the sections. Look at the Wiki code for another page to get a feel for how this works. You can find full editing instructions here.
Adding a Page
To create a new page, all you have to do is type the new page name in the "Search" box, press "Go" and then click the "this exact title" link on the resulting page. You will be given a blank page where you can put in your content.