What to Research
Obviously, you'll want to start with the product, service, or event you're writing about. What are the features? What are the benefits? What solution are you providing? Are there similar products, services, or events out there and how do they differ from yours?
What to research may also include a very specific topic, such as a particular health concern. But knowing what that health concern is, such as a disease, is not enough. You'll need to get to know the types of people who suffer from this concern, what their experiences are like--everything from symptoms, daily pains, mental health issues, to hospital and doctors' visits. What science and research already exists on this health concern?
Start with your general topic, the continue to get more and more specific, because the more specific you can be, the better you'll show people why they should buy from you, why they should hire you, or why they should attend your event. The more specific you are, the better you'll be able to address their concerns before they even ask questions.
Who to Research
You have to know your customers.
You also have to know who your competitors and potential partners are.
Additionally, it's helpful to know who else is in the industry, even if not a direct competitor, because you may learn more helpful information and even discover a few potential collaborators.
Where to Research
Start with a google search. Depending on what you're researching, academic sites may be helpful, but also sites like Reddit and YouTube. Research competitors' sites. Some of my clients are nonprofits, so I always like to research other nonprofits' websites, because others may have different, or more information. Sometimes others will have a better or easier way of explaining something.
Also, if you're able to and it makes sense, talk to people.
And believe it or not, watch TV. Listen to podcasts.
Something to note: sometimes, before even starting any independent research, I put together a creative brief based on questions I ask my clients to help me tackle the project. Creative briefs often help me figure out what I need to research or any further questions I may have. Most often though, I'll have done at least some basic research before this step though.