Hi Brian, you don't say what OS X version and Safari version you are using, so this is about Yosemite and Safari 8, but much of it may apply to Mavericks (haven't checked, sorry).
In Safari open Preferences > Extensions. Note that this is where you can turn individual extensions (or all) on and off, and uninstall them. At the bottom right of the window click on the "Get Extensions" button.
This takes you to Apple's Safari Extensions Gallery. Move down the web page and you will find a number of extensions including "AdBlock" with an "Install now" button. Click this, make sure the extension is active, and restart Safari. That's all there is to it.
Brian Tapper 