Yes offcourse a Biology student can join for B.Tech Computer Science there is no restriction

Biology student have the option of selecting either Medical or Engineering that's the added benifit of the course, since Maths, Physics, Chemistry is already there along with Biology which is also needed for Engineering course.

yes you have to attend extra coaching because the one who opted Computer Science subjects will be  have an introduction to coding. U can learn that in a span of one month or less if u have an interest

Hii sreelakshmi,

As engineering entrances test knowledge in physics,chemistry and mathematics. You must preopre well in these subjects. If coaching is possible, join it also. However I will suggest to not change subjects.

Good luck..