Look, at first you need choose your field where you are interested then you choose one particular exam where you want to prepare for. Like if you want take admission in top engineering colleges then you need to prepare for IIT JEE along wise there are also some exams where the syllabus is same like VITEEE, BITSAT, WBJEE, COMEDK, SRMJEE, AEEE, etc. so if you prepare for IIT JEE then you can also score good marks in these also as the syllabus is same.

Same for other streams also, you have to prepare for your aim and can give some other exams also with the same syllabus.

Good Luck :)