Most UI packages are made up of a collection of already available addons. This is just a matter of finding the bits and pieces and configuring them to your needs. The best way to do this is to go through a site like this and display the addons by category and then try and find the one in each category that works best for you.
If you want to start programming to make various addons yourself you might want to try some smaller addons, open them up and see what they are doing. Ask questions here and maybe try of the various WOW programming books.
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