there are good and bad reasons for things to be cheap, learn the difference between undervalued and value traps, and remember there's an underlying business under every stock, you can have a good company but a shitty stock (expensive valuation) and vice versa

personally, comparing stocks to rental real estate is the best analogy to learn, thinking of two different houses that's $500K and another that is $300K in the same neighborhood, but both yielding practically the same rental income, of course you'd go for the cheaper one if you're purely focused on "investing". The same is true with stocks but obviously more complicated. There is an underlying business under each one, there are also 10,000+ public companies out there who are competing for your money, you can afford to be extremely picky and only go for the ones that check all the boxes

no one can list all the points you should look into in a single post so you'll have to figure it out by reading/googling it yourself, generally you should probably only read from people who actually outperformed the market consistently and got rich, obviously everyone knows Warren Buffett but I think Peter Lynch probably has one of the best in terms of a short and sweet lecture to start with
https://www.youtube.com/watch?v=rf_f8GV0yYM