what should be my strategy to complete physics for neet 2021 strating from scratch ?

Neet 2021 syllabus is almost as same as NCERT syllabus of class 11 and 12. The syllabus with chapter wise weightage is as follows-

Physics-
Physical-world and measurement 2%
kinematics 3%
Laws of Motion 3%
Work, Energy and Power 4%
Motion of System of Particles and Rigid Body 5%
Gravitation 2%
Properties of Bulk Matter 3%
Thermodynamics 9%
Behaviour of Perfect Gas and Kinetic Theory 3%
Oscillation & Waves 3%
Electrostatics 9%
Current Electricity 8%
Magnetic Effect of Current & Magnetism 5%
Electromagnetic Induction & Alternating Current 8%
Electromagnetic Waves 5%
Dual Nature of Matter and Radiation 6%
Optics 10%
Atoms & Nuclei 3%
Electronic Devices 9%

However, there are chances that Neet 2021 syllabus will be reduced as the academic year has been delayed and due to this pandemic all educational institutes are closed so it is difficult to complete the entire syllabus and also CBSE has reduced the syllabus by 30%.  Please wait to hear from the authority about the actual syllabus.  Until and unless  NTA releases  the syllabus you can stick to NCERTs. For now you should start with NCERTs,  as these books are written in simple Language and easily understandable to all.

Other than NCERTs, you should also go for other reference books like HC Verma and DC Pandey for Physics. These books have standard complex problems which you should solve to get good score in competitive exams.