Fresh apples are kosher.

Like other fruit, if the apple in question is grown in Israel, you would need to ensure that tithes had been taken before you can partake. Also, if your tree is still young, you would need to make sure that the fruit is not from the first four years of its life, during which the fruit is considered orlah (or neta revai in the fourth year), and may not be eaten (this is not a concern for apples, which do not generally bear fruit before six years). Due to these considerations (as well as others), fruit grown in Israel is often sold under rabbinic supervision.