So I'm gonna give you the three addons I can't live without, and that's it. These are all the addons you actually need. (These are popular enough to receive upgrades soon after every new patch.) Can you (and do I?) use others? Yes, of course. But you can survive without others.
Auctioneer: If you plan to ever buy or sell things in the Auction House, you'll need to get auctioneer and run some scans so you'll know what you should charge/pay.
Gatherer: If any of your characters has a gathering profession (mining makes big bucks), gatherer marks the herbs/mines so you can go back to the same spots later and try and get more (nodes respawn in the same area, not necessarily the same node, about every 20-30 minutes). Recently updated with a downloadable database of nodes, so you can have all nodes immediately at your fingertips.
Titan Panel: More than anything else, Titan Panel is necessary for coordinates. Coordinates on the map make it easy for players to tell each other where to find quest goals. (Ie, on Wowhead.)
They ought to have directions, but in case they don't, or the directions are confusing:
- Download .zip file to desktop.
- Open My Computer from your Start menu.
- Go to Local Disk (C:)
- Go to Program Files (Ignore any warnings about changing/modifying contents -- that's just there to keep people from deleting the whole thing on a confused cleaning binge.)
- Find World of Warcraft folder, and enter it.
- Enter the Interface folder. You ought to see an Addons folder inside this one.
- Enter the Addons folder.
- Drag and drop the .zip files on your desktop into the Addons folder.
- Right-click on each .zip file and select "Extract here." This ought to place a new folder in your Addons folder. (If you cannot extract or "Unzip," you should download WinRAR.)
- Delete the .zip folders after you have extracted from them.
- Close and enjoy!