how much marks should I get to become an expert in software development?

Assuming you are in 12th grade, you need to get enough marks to get admission into Engineering college. Ideally you should try to get good marks in engineering entrance exams so that you can get admitted for Computer science or computer engineering. Developing computer science fundamentals and becoming skilled in computer programming will help you become an expert in software development.

Given that you are in school the first step would be to get an admission in a good computer science B.Tech program in your country.

While this is an important step it is also equally important that you start taking initiatives from your end early in your college life. Participate in as many hackathons as you can. You can also start competitive coding on platforms like Codechef, Hackerrank, Hackerearth etc. These are essential to develop an analytical thinking while solving a problem as well as improve your understanding of data structures and algorithms which are the building blocks of any successful Software Developer.

Try to work on your college projects diligently as they go a long way in clearing out many basics. Also , pick up a language be it C, C++ or JAVA and practice on it consistently so you have a fair hold on it while appearing for job interviews later.

Its not about marks. Marks is just the entry fee to get into your domain. What you should do is keep practicing more on coding. Participate in hackathon , various events which will give great exposure of coding. Try some freelancing stuff while you are studying in collage.