Hi aspirant,

To crack JEE main and advanced you need to have in depth knowledge of topics ask in examination. Generally students who are droppers start with those chapters which have maximum inter relevance with another chapters in any particular unit. So if you want to start with the 12th syllabus there is no better option then electrostatics which would cover various topics including capacitors, fields, circuit laws etc. Following this unit Magnetism  starts which requires concepts from electrostatics as a prerequisite for grasping concepts well. Also if you want to start with class 11th syllabus then Newton's law motion following which you would get chapters such as work and energy.

Hope it helps!!