It is simply the difference between simple past tense and present perfect tense. If a definite time in the past is stated or implied, then say "I was there (yesterday)". If no time is insinuated, then say "I have been there".

Both are past tense verbs and are fairly interchangeable. Have been is typically used to describe something that you have done in the past, such as "I have been to Mexico." Was is used to describe something in a more informal way, like: "I was just outside."