You can't change your branch after 1st semester. You can do it after completion of 1st year. You can do it if you score above 8.5-8.9 CGPA. This value is different for different board but it ranges between this only. So after your 1st year if your CGPA is in this range then you can change your branch for sure in your college.
1st year subjects are easy and you get mostly guided by the faculties. if you put your efforts and do good in your internal,assignments,quizes and tests and good performance in lab evaluation and atlast you write your semester with quality answers then you will get near to 9 SGPA. You can get many materials from online also. if still you understand then you can ask your teachers too. They will help you to know.
Hope it helps!