There are 2 main schools of thought are Fundamental Analysis and Technical Analysis. Usually people will fall into one or the other and become knowledgeable within that category. As a start, maybe you can figure out which of the two makes sense to you.

Fundamental analysis is valuation of a company usually by cash flow and earnings. If you can value a company as a whole, you can estimate the price its stock should be based on how many shares of the stock exist. Then the idea is to invest in stocks that are undervalued.

Technical analysis is the use of charting patterns with the assumption that movement in the market happens in some sort of cyclical nature. Certain formations on the stock chart may produce a high likelihood of some future movement. For example, some people say a "head and shoulders" pattern indicates a reversal of direction.

Over years of trading, I've found that there's so many different ways that people are making money (and even more ways that people are losing money). There is no single best way. Find something that works for you, become an expert, and exploit it!