Let's sum up the opening principles: take the centre, develop your minor pieces, castle to keep your king safe, don't bring the queen out early, don't move the same piece twice. Now three questions.
Question one: White has developed their pieces. What is the most natural move now?
Question two: White has put two pawns in the centre. Which piece do you develop?
Question three: you are Black. White is attacking f7 again. Stop the mate.