Your wiki wordmark is probably one of the first things your visitors will see. Therefore, it should best match the rest of the layout of your wiki, as well as to some extent represent the wiki content: girly wikis might contain curly letters whereas gaming wikis might match the game name's color and font.
In order to change the wordmark for Wikia skin users, you first need to go to your wiki's Special:ThemeDesigner page. You can do this either directly in the URL bar or by accessing it through the admin panel.
Entering the Theme Designer, you will find yourself in the tab Theme. Go two tabs below this, to the tab Wordmark. For now, you will need one of the two left tabs: Text wordmark or Graphic wordmark. If just text is fine with you, choose Text wordmark. If you want to upload an image, choose Graphic wordmark.
Step 2 - text wordmarkEdit
For a text wordmark, stay in the most left third of the Wordmark tab. First, type in the text that you want to show and click the button "Change text". Then, pick the right font and font size that you think suits the wiki name best. Depending on how picky you are, this may take a bit of trying and testing. Keep in mind that it is best to keep the name close to what the wiki stands for!
Step 2 - graphic wordmarkEdit
To pick a graphic wordmark, first you must have an image. You will need a .png file of less than 250px wide and 65px high, of course keeping copyrights in mind. If you have this, click the button at the top to browse your computer for the image. Once you have picked this file, click "Upload". The image will soon appear as a preview.
Check what your wordmark looks like on the page itself, right below where you were working. If you are certain of your choice, click "Save, I'm done". The Theme Designer will bring you automatically back to the wiki's home page.