when should i join coaching to give the enterance exam for ca after 12th?

Of course, it is your choice. But I like to advise you that, currently you try to gather information from seniors who have gone through it on 'How, where why and when'. It will be better if you are equipte and realise about on it from now on, please.

first complete your 12 than follow up their website

entrance exam is compulsory for banking and finance.....?????

Hi, It has to be as per your convenience. The basic thing you need to be aware of is the significance of the exam. You require a focused plan and for the same create a time table. You can separate each subject and study accordingly. You can take small footsteps and achieve a lot within a small time