JEE Main

Hi,
It's great to see that you have discovered what you wish to do. My suggestion would be to analyze your skills in each of the field. By your question I assume that you are still in high school, in that case you might not be able to analyze yourself with respect to mechanical field. So start by analyzing yourself with respect to computer science like how well are you at programming (C, C+) and what's your proficiency in various computer applications like MS Office (Word, Power Point, Excel, Access, etc.), Media editing (audio &/or video), modeling and animation, etc. If you are good at it then you should go for it.
Being a mechanical engineer let me tell you that mechanical is a very vast field, in fact it is a combination of all the fields be it computer science, automobile, thermal (heat transfer, refrigeration and air conditioning, thermodynamics), chemical, physics, environment, supply chain management, etc. In order to test yourself with respect to mechanical make sure you are good at thermodynamics (basics are introduced in chemistry), mechanics (moment of inertia, newton's laws of motion, centroid, stress, strain, etc.), mathematics (especially differentiation and integration). These are some basic topics that are introduced at school level, which later in graduation and post-graduation gets elaborated.
PS: in high schools people generally have misconception that the mechanical field means just designing of automobiles and all. In reality mechanical is much more than that. In my opinion none of the fields can survive without mechanical field's support.