There are two ways to conduct product research on Amazon; manually or automatically.
You can do it manually by checking the Amazon best-sellers category, which is a great place to start for substantial product research. Other manual ways include looking at Amazon storefronts, exploring Amazon Basics products, looking at product sub-tiers, and scouring the "Customers Also Bought" section.
Automated product research can be done with tools like Sellerapps, MarketScout and Jungle Scout, which are outsourced methods designed to give you an estimate of monthly sales, revenue and number of product reviews for different ASIN categories.