If you are new to Google Display, I recommend you start with automatic placements. This is where AdWords shows your ads on relevant websites based on your keywords and other factors. You have the option to simply opt a search campaign into the Google Display network, however, that is not the recommended setup. I recommend creating a new AdWords campaign targeting Google Display only, so you can allocate a separate budget and customize other settings for display.
Decide how much of your budget you'd like to allocate for testing display. Start with a smaller budget until you evaluate if display will work for you while you optimize your campaign.
