At what stage I should take coaching for CA?

If you are able to handle your SSC and your sole aim is to do CA then start going for the coaching of CPT exam. Otherwise, after School also you can go for that course. Most of the students go for this option. Also, if you want to do Graduation alongside preparation of CA then there is always an opt

Depends upon in which place you live. After knowing your place, I can suggest you the best place of coaching in your area. Firstly you will need to appear for foundation programme then intern programme and then at last there will be a final programme.