Great to know about your interest in Software Engineering. The first steps should include getting a strong hold on a programming language. I would suggest that you choose Python because of its highly detailed documentation and community online. You can then move forward to master other programming languages and develop applications for mobiles and personal computers.
Software Engineering isn't just limited to programming though. You need to learn a lot about what goes behind the execution of your programs and scripts and a good way to learn that is pursuing B. Tech or B. Sc in Computer Science. A degree will bring you much closer to your dream of getting a job as Software Engineer in a big company.
One thing that you should keep in mind is keep practicing and learning. Internet is full of study material and you can teach yourself to become an expert on anything. Here is an inspirational video about a man who taught himself how to code.
You are asking this at a great time in your life. I'm not completely familiar with the education system in India but it appears 2nd PU is a set of pre-university courses that would correspond to American high school.
In the United States you may choose to attend a university and obtain a computer science degree. This would be about a 4 year set of courses and expose you to a variety of technologies and help you develop the skills you would need to learn on the job.
Another path would be self-learning. If you choose this path you become responsible for determining what you need to know. You may pursue tutorials and online learning resources.
Either way, an internship is an excellent way to gain exposure to technologies and develop basic working skills and a network of contacts to help you land a job after you complete your course of study. I had an internship during my 4th year of college that transitioned to a full-time job as a software developer upon graduation.
Give your best in CET exams to get good ranking which will fetch u seats in good colleges. For software field, your PCM combination would be important , so work on that. Chose a branch which you think you might be good at. Strive hard, study well.. Good luck!
Once you get CET score, based on score you will be allowed to select engineering course in college. you need to select computer science or information science streams in Engineering. After 4 years of engineering, you will qualify for find a job in software industry. Please try and score high in engineering, so that you can find job easily.