I have a lot of conflicting opinions on the matter, but I will say the one thing you must do is to do your homework. Make sure you research and understand the implications of choosing one company over another, how they generate cash flows, where the industry is heading, momentum and behaviour, and read up on both theory and empirical data on stocks and the market. Please, I implore you, do not be caught by flashy gimmicks with promises for high returns, hot stock tips from friends (or worse yet, people you don't know) and the like. At the very worst, you'll come out with a practical application of your knowledge and you can say, 'hey, this was the best information I had at the time and I made the most informed decision I could'.