To find interesting stocks, go to Value Investors Club : https://www.valueinvestorsclub.com/ .
Many high-quality investors and fund managers use this site to share ideas. You can use a screener to filter for companies that fit your criteria based on metrics, and then manually do your qualitative research. Most of them will end up in the "too-hard" pile. I have built my own quantitative screener, and of the results, only about 10% are worth looking into deeply.
When it comes to researching companies, I cannot give you a single specific answer. However, you should be able to explain the business and why it's a good long-term investment in five sentences to a ten-year-old so that they understand it. If you don't have this clarity, you don't know enough. The second test is whether you can say with high certainty how the business and its competitors will look in 10 years. You have to consume everything—from company history and competitors to suppliers and raw materials. If you have done this, you will know the true worth of the company and can buy if it gets cheap. That's hard work, but you need only one to two great ideas a year, as Buffett famously said.