You should consider starting your CA tuitions for the CA CPT foundation course as soon as you give your class 12th final exams.
It is advisable to join coaching as soon as 12th Results are announced, if you are writing foundation. If you are writing Executive after graduation, join as soon as you finish your graduation. Failing to plan is planning to fail.
It normally takes 5-6 years to finish off the course with disciplined study pattern and with proper control over the tempting distractions.
Cracking CS examination is definitely difficult but it's not impossible. There are lot of Acts, Laws and statutes to master but with proper dedication, determination, written plan and schedule, cracking this professional exam will be easier.
Coaching is optional but in this competitive field with variety of subjects and majority of which is statutes and different types of concepts, it's better to get trained under the academy as the concerned faculty will do the background work on every subject and give the best solution to you rather than you trying it.