If I was and if I were are both are correct but are used in different situations.
If you are using the second conditional to talk about unreal situations then use if I were.
- If I were you, I would go to Europe.
- If she were the manager, this company would be better.
If you are talking about real past events then use if I was.
- If I was late, I usually took a taxi.
- She visited Gary if she was in Liverpool.