Hii Nishita!

You haven't mentioned the subject stream you are currently studying or want to pursue in the St. Xavier's College, Mumbai. If it is BMS, then the topics which you need to prepare are economics, general knowledge, current affairs, logical reasoning, creative thinking, language etc., You need to pay attention towards the apptitude questions also. In order to make your position in the merit list, you need to score well in class 12th and also perform well in the entrance exam. Prepare a timetable and follow it strictly. Practice as many previous year question papers and question banks as you can.

Hope this helps.

Good luck!