B.Com

Dear Mohammed Faziz,
I am very much glad to know that you have really great, different and adventures ambition. See, to join ATS, you don't need to clear any entrance exam because they have their own procedures and criteria to select a candidate for the team. Their selection procedures mainly based on your academics. They don't bother about your stream. But you must go for civil services exams like UPSE or any other defence organisation's exam.
Your physical fitness. I don't think that you need any explanation on this.
Your past records of performance. It shows that how effective you are and your capabilities to work under extreme and pressurised conditions. Based on theses factors they automatically pickup the candidates and offer them positions in extreme defence sectors like RAW, IB, Military Intelligence or ATS.
My suggestion is that you must have to prepare for civil services exams (most preferred NDA but you can't as you are a commerce student) like IAS or IPS. Try to get into army and they will pick up automatically as their is no criteria to apply individualy.
Hope you find this helpful.
Kindly upvote the answer if you feel satisfied.
Ask any query any time.
Best of luck dear.