For becoming CA you will have to enroll with ICAI after passing class12 and appear in CPT exam.

Next level is IPCC exam in which you can appear after qualifying CPT exam.

After that you will have to undergo articleship training under practicing CA.

Final step is Final exam in which you can appear after training.

For preparation :
First you should have thorough knowledge of complete syllabus.
Make a proper study plan.
Try to prepare notes on your own.
Practice more and more previous year question papers to understand the pattern and types of question asked in exam.
All the best

First, make sure you know the whole syllabus before starting your preparation. Try to understand the topics don't just mug up the things, You should be determined to crack this exam a lot of hard work is required. You can make your important notes it will help you in revision. Try to solve as many as mock test series for your practice.