Your question needs some extra information:
- if you use " which" you must have a limited number of something : e.g. which sport winter or summer, which team sports, which school subject, which month
- If there is a question form -> inverse the order, so instead " you are good at " , guestion form is "are you good at?"
- and prepositions - you need just one: 'AT' or 'IN' but not both
So the question:
Which summer sport are you good AT?
answer: I am good AT swimming.
Which month were you born IN?
answer: I was born IN October.