One of the most difficult aspects of learning English is to learn which prepositions one must use in which context. Often prepositions can be found in counter-intuitive contexts. An example is ''I'm on the phone''. At first glance, this doesn't seem to make sense at all. How can one be ''on'' a phone? It's not like you can stand or sit on a phone! But of course, in this context the use of the preposition ''on'' simply implies that you are taking a call. You are speaking to someone else ''on'' the phone.
Another example of counter-intuitive preposition use in English is the preposition ''at'' in the context of ''I am good at English''. Usually at is used in the context of place: I am at home or at school. But in this case, the literal meaning of ''at'' is to have a kind of orientation towards English, as a compass points towards a certain direction. When you say, ''I am good at English'', you are implying that in a certain sense, your own ''compass'' points towards the fact that you are good ''at'' English.
There are many other counter-intuitive examples of prepositions in English. As you can see however learning prepositions is a difficult task for a non-native English speaker. The best advice I can give to a non-native English speaker about learning which prepositions to use in which contexts is simply to read more and learn the different usages of different prepositions.