A number of general things that you can do on the wiki. For the most part, bulking up pages with useful text is acceptable.
Improve a Stub
You can help by improving a stub. A stub means that a page is incomplete or doesn't have enough information (typically if it is under 300 bytes in size). Once you've improved a page like this, be sure to remove the template that marks it as a stub.
See a list of stub pages here:
Fix Red Links
Red links indicate that the page that the link is supposed to direct to doesn't exist. Create new pages for these links. If you think the item that the link describes does not merit a page, eliminate it or link it to a more general page. For example, "Dol Guldur Shovel" can be linked to the "Dol Guldur Equipment" page.
See a list of red links here. Note: Do not create a page for "*"! Several redlinks like this are only used here to have emoticons without producing lists.
You can also help by adding images to pages without images. Whenever you see this:
it means that this page needs an image.
Help prevent orphaned pages by putting them into categories, which are similar to tags. This can be done at the bottom of the page. For example, the page "Wood-Elf Warrior" can be put into the "Mobs", "NPCs", "Elves", and "Woodland Realm" categories.
See uncategorized pages for a list of pages in need of categorization.
What to Do (click to expand)
There are many specific things on this wiki that need improving, more than just the things below; see here for a more complete list.
If you need help with wiki coding, there is an article on the wiki that can help you with that.