I'm studying in 12th commerce now so should I give coming CPT exam i.e 16th dec 2018 ?
Sorry to inform you but you can't appear in CPT December attempt because you haven't completed your 10+2 yet. Once you complete your 10+2, then you can go for CPT or other examination.
Till, focus on your 10+2 syllabus. It will play crucial role in CPT.