The first step is to decide on an addon to get.
Some good places to look are World of War [dot] Net and Curse Gaming.
I will assume that my astute readers will now have the addon saved to their hard drive. Open it, using either Winzip, the built-in ZIP extractor from Windows, WinRAR, or whatever you Macintosh people use to open ZIP or RAR files.
From here, open up the Folder where World of Warcraft is installed. Then go to the Interface folder, and the Addons.

You’ll notice that each Addon has its own folder. This is how it differentiates addons.
Now load up WoW and go to the Character Selection screen.

Look in the bottom right of the screen. There’s a button that says “Addons”. Click it. Yes. It’s okay. No, I know you’re not normally supposed to click big red buttons but this one’s ok. I’ll wait.
Done taht? Good. This is a list of all the addons, WoW recognizes.

Ok. I’ve boxed in red the important things here. The first thing to notice is the “Configure Addons for:” option. This will let you set Addons for every character, or just one character. If you wnat to set addons for two characters, you’ll have to configure them individually. Second is the “load out of date addons.” This involves addons that have not been updated since the last patch. By default they’re turned off. You can check to see if they work or not by using this feature. The last thing to notice is the checkbox to the left of the addon. This is whether or not it’s turned on. Check box means on, no check box means off. And the buttons at the bottom let you enable or disable every addon listed at once.
Now just load up your character and play with your addon assisting you.
For suggestions on addons, you can filter to the Addons options through the Category navigation.