‘At’ refers to things being literally (and sometimes physically), at the beginning or end point of something; e.g. at the end of the journey, film or a street. ‘In’ refers to something in the beginning or end of a process, or something else that is more abstract in nature.
Prepositions must have an object, and so, if it is a verb, it has to be -ing; e.g. You must get used to seeing things in a new light.
In questions, prepositions usually come at the end of the sentence; e.g. Who are you arguing with?
