Dear student,

First of all you must mention you field or stream that from which stream you will be passing out. Assuming that you are from science stream, then if you want to pursue your career in engineering then you must have Mathematics as one of your subjects.
And if you want to make your career in medical then you must have Biology as one of your subjects.
Now if you want to pursue engineering after class 12, then, first you have to give the exam called JEE Mains.
JEE mains is a national level entrances examination for students who want to pursue undergraduate courses in NITs, IIITs or GFTIs. If you qualify JEE mains exam then you will be eligible to take admission in these reputed colleges and also you will be able to give JEE advanced exam which will be helpful for getting you IIT, if you secure a good rank in JEE advanced exam.
Now suppose that if you haven't qualified JEE mains exam then no problem, you can give many Engineering entrance examinations after class 12 like the WBJEE, upsee, viteee, comedk, cusat,  bitsat, etc. and other state Engineering entrance examinations.
If you are from Medical stream then you can give the entrance examination called NEET and other state medical examination, to get admission in government and private medical colleges.
So you have wide variety of entrance examinations after class 12 but before giving all these exams, first you have to pass your class 12th board exams.