You can use the command '/option flash_test_gui true' in order to open up the GUI debugger, which will allow you to customise most of the standard UI elements. You will want to be editing the '_root' definitions, so if you're currently seeing '_global' you will need to click the '_root/_global' button at the top of the GUI editor.
Like many other commands entered into the chat window, you can type /option and press TAB to see a list of the options you can set. You can see a list of many of these options and their default values by browsing to your Secret World installation folder and finding the file Data\Gui\Default\CharPrefs.xml. The default installation folder is something like C:\Program Files (x86)\Funcom\The Secret World
Any changes that you make to the default settings will be saved in %localappdata%\Funcom\TSW\Prefs
There is more UI customisation available, for a more comprehensive guide try here.
In addition to UI customisation, The Secret World supports addons from websites like Curse - these are reasonably well covered in Ingmar's answer.