Maybe a useful way to look at it could be to think of が as meaning"the". So, as Rumina previously mentioned, わたしが would emphasize the fact that it was you. "I am the one who ate the apple". Versus regular old は, just stating that "I ate an apple". Good luck! This question will keep on coming up for ever and there is no simple, universal answer!

- What/Which did you eat? - I ate the apple. [= It's the apple that I ate.] (私は)りんごを食べました。 - Who did you eat the apple? - I ate the apple. [= It's me who ate the apple.] 私が(りんごを)食べました。