If you are unfamiliar with the way addons work, then my first suggestion would be to try an "all in one" ui such as nUI, which can be downloaded
here, or TukUI from TukUI.org.
Both of these work in much the same way, ie they both set up your action bars, unit frames etc., albeit in very different ways.
If you decide to take the do-it-yourself approach, just remember that setting up your own ui from scratch can be a very daunting proposal, especially if you've been playing WoW with just the defaults for four years. If you decide this is the route you want to take, then I suggest you start with the most basic pieces first; Find addons for unit frames, action bars and possibly a minimap replacement to begin with. Experiment with those. Place them in a way that works for you and don't worry (at least, to begin with) about how it might look to someone else. The most important part of designing a ui is setting things up in a way that compliments your play-style and not what someone told you it should look like.
If you run into problems (don't worry, you will) ask your questions here, and you will almost always get a fast and polite response. WoWI is
the place to go with questions regarding individual addons and ui setup.
Good luck and have fun!