Hi Daniela, preposition are complicated in Italian! I try to explain to you the use of A and IN when you speak about places.
Preposition A is used after verb that expresses movement. For example " Io vado A casa"= I go TO home (it's the same of English preposition TO).
A is used also when you tell about where you LIVE or STAY "Io vivo A Roma" = I live in Rome or "Io sto A casa"= I stay at home.
These are some examples, I hope they're useful!