How do you recommend deciding which stocks to pick?

Okay so I know a little bit about stocks, but not a lot by any means, and would like to understand:

1. How market cap is determined (originally I thought it was just Market Cap = Stock Price x # of Shares) but found out there's way more that factors into it (reason I care is because I heard it's a good indicator to predict future prices)

2. Just curious how other investors decide on which stocks to pick? For example, I saw that QQQ, VTI, VOO, & IVV all have done about 10% per year, the last 10 years, and since they're index funds, figured they'd be somewhat safe) but then saw some like Apple, Microsoft, & Nvidia have done up to 27% per year for the last 10 years and want to make the most of my money so I can retire someday (currently 29 years old)

Additional Info: Grew up poor, with next to no financial education and just trying to learn so I can have a good financial future, and pretty much just try to self educate with Google and YouTube (like Graham Stephan, Andrei Jikh, etc). But sorry if I got any info wrong just trying to learn.

Thanks in advance