can i start CAT preparation now to target best B school?

You can, but now when July is almost ending you need to focus on covering high weightage topics more than covering everything. At the same time you need to put in more hours balancing each section equally. Try brushing up your concepts and give one full length mock, with that you'll know which is you strongest section, try taking your strongest section to the moon.
Start giving topic, sectional test to know where you lack.
All the best!!

If you have zero basics and who want to start from scratch then It was harsh to say but from now it's not possible if you are working. But if you have full preparation days which are left then you will take a chance