In oral わたしand あたし same meaning. わたし is slightly politer than あたし. わたくし is the politest. to company bosses, I say わたし or わたくし to friends and families, I pronaunce わたし or あたし. it's casual. but men hardly pronaunces あたし. it's strange. When the male says わたし or わたくし, It's　polite. Casual way for men, おれ（俺） or ぼく（僕）. In written 私(watashi or watakushi)　is correct　and formal. わたし is not wrong, It's casual. あたし is informal and a little childish. I hope my response is useful for you.

Thank you!!! You previously asked me to fix your English if there is any mistake so instead of "You've already get a perfect answer" it will be "You've already GOT a perfect answer"...^_^

You've already get a perfect answer.