Hi Nada, these words are prepositions. It often depends on what you are saying. For example in and at can be used as prepositions of time or prepositions of place. ( I travel in December or The cake is in the tin. We go out at the end of the week or He is at the cafe).
Time
In: we use in to refer to a general, longer period of time, such as months, years, decades, or centuries. (In December, in 1990)
At: we use at for the most specific times, and for holidays. (At lunch, At Christmas, At 11;30)
For: used for saying what happened or existed during a short period before it changed. (For 2 weeks, For five years)
Place
In: we use in for the largest or most general places. (In South Africa, In Asia, In the river)
At: or exact addresses or intersections, we use the preposition at. (The the post office, at the corner of 5th Avenue)