Strictly speaking, 'were' is the more correct form, but 'was' is also acceptable. You can use either of them in most situations. However, if you are taking a formal grammar test, I'd advise you to use 'were'.

Grammatically, using "was" would be incorrect. "If I was..." can perhaps be heard in spoken English, but it just sounds very clumsy and awkward. If I were you, I would always use "were".

Renas: Su Ki is correct. I would also add that the "was" form is not exactly correct grammar. In the United States, you might be more likely to hear the use of "was" if someone was hostile. For example, someone might say to someone they did not like, something like this: "If I was you, I'd just keep on walkin!" It shows hostile intention. But then also, Americans will sometimes use this as slang, when they play with their language and adopt an affectation in speech. I might say to you, as your friend; "Renas; if I was you, I would take that job!" It would be a more grammatically correct and polite speech to say; "Renas; If I were you, I would take that job!" It is not unusual for Americans to adopt a "country" or a "countrified" affectation in speech. One hears this all the time in some movies about "country folk".