i wanted to become a software engineer? which subject i must choose?

All you have to do is to complete BE IT(Computers)

Well, first of all its great that you have planned you future goals at this early stage. Not everyone does that this early. At your stage students are only focused on how to score maximum marks in exams/boards.

Now coming back to your question. So as you have decided that you have to become a software engineer I assume that you have taken Science in your 11th along with Computer Science as an optional subject. Now that's a must.
In order to become a successful software engineer in the future you should have strong inclination or you can say, INTEREST towards computer science or computers in general.

Throughout your 11th and 12th read computers with complete focus, dedication and interest (I'm not saying that neglect other subjects but focus more on computers). This is the time when you can build you fundamentals from ground up. Its like when you start to learn a new language they first start with alphabets then words then sentences then nouns, tenses etc. That's what is going to happen in your 11th and 12th. You will learn the fundamentals of software engineering in these two years.

So as of now just focus on the subject Computer Science and dont just rely on books prescribed by the school. Browse the web there is almost everything available out there these days.

So as of now just focus on one subject that is Computer Science, learn the basics and If you have to then explore further on the internet.

software engg is really good professional will fetch you lot of money ,but if you want to be software enng you should be logical good, you need to have computer science btech graduate or similar qualification if your want earn good revenue in software field you lot other stream where you will earn lot of money ,but if its your passion then you should defiantly go for it