"If I were/was you, I would spell 'I' with a capital letter" ;-)
Both are acceptable, but "If I were" is better, especially when writing.
When saying "If I were..." you should never use "will", because it's an unreal conditional (I can't possibly be you...I am me!). You can use would, could, should, etc. but not will.
Long answer (this information is not important, but you might be interested):
English has a "tense" (technically it's called a "mood") called the subjunctive. Normally, it's not important to think of the subjunctive when learning English, as the past and present subjunctives are the same as the past and present simple. However "be" is a rare exception: the past subjunctive of "be" is "were", no matter what the subject is. Many languages (eg. Spanish and French) have very common and subjunctive moods that are difficult and important to learn, but as an English learner you can (and should) completely ignore the idea and think of all subjunctives as past and present simple. Second conditional "be" is the exception, but even in this case many native speakers drop the subjunctive and use past simple: "If I was..."