This evening was the scariest evening of my life! It started out/off OK. Tom and I were sitting at home, watching a game show. Then someone started knocking on/at our door.

Do both started out and started off sound right and convey the same idea? If yes, could I replace them with "In the beginning, every thing was OK?"

In addition, what is the slight difference between knocking on and at our door if there really is difference?
No significant differences, and all are OK except that everything is one word.
Thanks, Mister.

Got it.