Strictly speaking, the correct form is 'If I were...'. It's a past subjunctive. However, the 'was' form is so widely used nowadays that it is seen as an acceptable alternative, in all but the most formal of contexts.

"If I were... I would" is correct, and considered "better" English. If I were you, I'd use it. However, "was" has been so common for so long that it is also accepted. As far back as 1921, H. L. Mencken was complaining that 'All signs of the subjunctive, indeed, seem to be disappearing from vulgar American. One never hears “if I were you,” but always “if I was you...."

"Were" is the original past subjunctive form of "to be". The past plural of "to be" changed to the same spelling after several hundred years. In modern English, it's becoming acceptable to use "was" as a past subjunctive, maybe because the past simple and past subjunctive of all other English verbs look exactly the same. However, "were" is the traditionally correct form of the verb.

You can use both sentences. "If I were" is used more often, but both are correct. --> If I was you, I would read that book. If I were you, I would read that book.

If I was..., I would ... were is using for plural such as we were , you were, they were