There’s no one ‘defining factor’ that everyone considers before making an investment decision. It varies from individual to individual and what suits you may not necessarily suit another.
You could be someone who follows technicals, someone who follows fundamental analysis or you could just be someone who invests with a gut feeling. Whatever be the theory behind your investment, what I wish to say is that there are no set rules you have to follow to single out stocks and pick them for investing.
I suggest you go through the basics of stock markets, fundamental analysis & technical analysis on Zerodha Varsity here: http://zerodha.com/varsity/ and see which style of trading/decision making helps you.