'If I were' is in the subjective mood, 'if I was' is in the indicative mood. 'If I were' is correct in the vast majority of cases. A verb is in the subjunctive mood when it expresses a condition which is doubtful or not factual. It is most often found in a clause beginning with the word if.

I say "If I was". This may not be grammatically correct according to the rules of subjective/indicative mood but this is how I speak and how most people I know speak. Saying "If I were" is perfectly acceptable to my ear and doesn't sound the least bit awkward so maybe for you the answer is "both correct" because nobody will notice or care.

Thank you for the comment. Well, as I said before, my English grammar taught me I can use whichever I want, because they're interchangeable (at least I understood that). However, I see people often using ''If I were'' instead of ''If I was'' in American English, so Iwill start to use it more frequente. Could I do one more question? In an official exam like Toefl or Ielts, Would I lose points If I used one instead of the other?

A great debate! I agree with all really - some say when written use 'were' but often, especially with British English and in speech, we use 'if I was'. There is something slightly stunted and archaic about always saying 'if I were'. If I were you, I'd use whichever you have a feeling for, however, if I was you, I would probably not quite know which feeling was correct! :O On the safe side 'If I were', on the dangerous side, experimenting and ask us for specific examples :)