The first one would be ok for "I ate an apple today" for the answer to "What did you do today?". \^o^/

The two sentences convey the same meaning in a very general sense. The first suggests that the point is that it was you who ate the apple, whereas the second suggests that the main point is that it was done today. The two sentences translate more or less as "(As for me,) I ate an apple today," and "(As for) today, I ate an apple," respectively.