What happens if I tap Aetherling to attack and in my 2nd main phase I exile it?
you are correct.

as long as you activate the exile ability before your end step, it will return untapped before your opponent's turn.

It does not return instantly however, if you exile it during your turn e.g. during the combat phase, it will not return until your end step. If you wish to cast sorcery or creature cards with it on the battlefield, you will not be able to until your next turn.