AdBlock Premium starts blocking ads from the moment it's installed in your browser. There is nothing more you need to do.
There are some optional features you may want to try out, however. For instance, you can allow ads on individual YouTube channels to support your favorite YouTubers. You can subscribe to additional filter lists to customize your browsing experience. You can sync your filter list subscriptions and other settings if you use AdBlock Premium on more than one computer.
To find these features and other settings, click the AdBlock Premium button in your browser toolbar and select Options.