How do I decide whether to use, "If I was" or "If I were"?

It's 'if I were' for hypothetical in the present or future, and 'if I was' when talking about something presumed true in the PAST. Look at this sentence: "If I were a bird, I'd fly." Here, the person is just making a wish. This is also known as 'subjunctive mood'. The subjunctive mood is used to express a wish or possible situation that is currently not true.