Hello dipanwita.
Well, I'm sure you know that the biggest exam for engineering in India is JEE mains and JEE advance.To prepare for these exams,the best way is to take a coaching for these.You should start preparing right after class 10 and be thorough with PCM.It's not that tough but you need to work hard for it.Choose right books and study materials.There are other exams too like BITSAT,COMEDk and many more but the syllabus is same.Hence, preparing for JEE will work for all.Hope this helps.
Good Luck!