I get 50 percent in 12th in science stream which courses is best for me after 12th

See marks don't define which course you'd be successful with. Your boards marks are a result of theoretically knowledge that you were supposed to possess. You might not have had the proper chance to study what you liked.

For example a student might have scored 60% in boards but his bifocal subject might have been electronics and in that he scored 180/200 that doesn't mean he shouldn't select tough subjects he clearly understands electronics and must choose electronics from which ever college he gets. For your case close your eyes and decide what is your favourite subject if it's chemistry go for chemical see liking what you learn will make you a great engineering.

If you think you can't handle the pressure of btech go for bsc it's a bit simpler and it finished in 3 years also take bsc if you haven't cleared any entrance exams

Else try btech . U can choose bsc it or bsc in computers same with btech IT/Cmpn/extc/Etrx etc etc

Currently it and computers are having high demands and mechanical is always available as a good alternative if you are  good hands in work.