Staart right now.Infact this is the best time to start for jee preparation,august would be too late!!!You see time is very precious for any competetive exam.Estimate time you are left with,analyse the syllabus,watch out your your approach towards your preparations,what resources you can use and plan out your road ahead.
 
Too many books just increase pressure and frustration because there are always too many of them.HCV is the best book of physics espesially the small answer questions and the MCQs should be done very very deeply and discuss with your instructor at coaching or school.I.E Irodov too is a good book which stress more on concepts than on calculations(they dont make you do calculations in JEE).
which objective mathematics you are talking about i dont have any idea,i mean publication n writer.Follow what is asked in your coaching.Same for Chemistry.
 
About your marks!!!, they are just not that bad.If you have not yet started formally with jee preparations then you dont need to worry.the questions are just different than what you saw in schools and ofcourse have shown big improvement.And that is most important,just keep it up!!!