Before sitting down at the poker table, you have to be able to answer one fundamental question: how are you going to win money by playing? Your answer is your strategy. Without an answer, you don't have a strategy.
Poker is a game of exploiting the mistakes of others. If they don't make mistakes, you cannot gain an advantage and make money. When planning your strategy, the first thing you need to consider are your opponents and the mistakes they are likely to make. Alternately, you can seek out opponents who make particular types of mistakes that you know how to exploit.
You don't need to have a fully formed strategy before you sit down. Your strategy can—and should—change throughout a session as you learn more about your particular opponents. For example, you may notice that the player to your left doesn't defend his blinds. You then alter your strategy to win money by stealing his blinds when its folded to you (but not so frequently as to make it too obvious).