To find most of my addons I find through curse.com. Curse has a very handy client for updating your addons nearly automatically, it tells you when updates are avaliable for particular addons and asks you to select it and click the update button.
installing curse client- I get most of my addons from curse and like most of us I dislike updating addons manually. Curse client updates addons at the click of a button and tell you when updates for your addons are avaliable. You can find Curse client Here.
installing addons manually- If curse doesnt have the addon you want in its database you can go to the old school by installing addons manually. You do this by downloading the addon, opening the zip file and extracting it to C:\Users\Public\Games\World of Warcraft\Interface\Addons for vista or C:\Program Files\World of Warcraft\Interface\Addons for xp or Applications\World of Warcraft\Interface\Addons for mac
load out of date addons- some addon developers do not update their addons with every patch and to use the addon you will need to check “load out of date addons” by clicking on the addons button on the character login page (in the bottom left hand corner) and then run the game as usual (you will not have to do this every time you load the game)
slash commands- slash commands are a non GUI(graphical user interface, ie buttons etc..) way to open configuration screens or configure addons. These are particularly helpful if you cannot find the button to open the configuration screen for an addon or want a faster command line esque way to open or configure addons.