If I have not studied computer science properly in 12th, should I take a B.Tech in CSE?

Yes you can opt for computer science engineering . Computer science engineering is a very growing and demanding field . You don’t need to worry if you have not studied properly during your 12 th class. You can start learning programming even without beginning your college. Just pick up any programming language and start learning .there are plenty of online free and paid resources for learning like udemy,Coursera e.tc so just learn the fundamentals and you will start developing interest and passion for coding and you will be at upper hand when you will enter the college so start learning programming and always try to improve your skills every day and try to learn new technologies .