You can use either. "If I were..." is more correct and more formal. "If I was..." is very common and informal. If you like music, you could look up the songs: "If I were a rich man" (Fiddler on the Roof), "If I were a boy" (Beyoncé) and "If I was..." (Midge Ure).

It depends on the situation. 'if i were' is technically more grammatically correct but 'if i was' is more conversational and relaxed.

You may use either if the premise is contrary to fact, and you are using a conditional construction, e.g. "I was/were rich, I would move my money to Panama". In that case, "were" is more common in US English, and is also fine in UK English, particularly in formal contexts. But otherwise, and if you are talking about something in the past, you need "was", e.g. "If I was unclear, I can explain it again".

I agree with both of the other answers. My advice, however, is to say whichever one you feel like saying. This is a stylistic decision. If you only spoke 100% correct and proper English, you would sound 100% correct and proper. Being 100% correct and proper isn't native, and therefore situations like this where you could use either and be colloquially correct, just use them interchangeably and mess around with which one you like more. Cheers.