In the order to become Company Secretary after 12th commerce I would suggest you to do Company Secretary Course. This course has following three stages:

Foundation Programme
Executive Programme
Professional Programme

Eligibility:
The applicant should have passed 12th with Arts, Science or Commerce stream from any recognized board of India.

Fees:
The fee for CS Foundation Programme is - Rs. 3600/-
for Executive Programme - Rs. 7000/- for Commerce Graduates while
- Rs. 7750/- for Non Commerce Students

Subjects for CS
There are 4 papers in CS Foundation Programme

English and Business Communication
Economics and Statistics
Financial Accounting
Elements of Business Laws and Management
There are 6 papers divided into 2 Modules in CS Executive Programme

Module I
General and Commercial Laws
Company Accounts, Cost and Management Accounting
Tax Laws

Module II
Company Law
Economic and Labour Laws
Securities Laws and Compliances

There are 8 papers divided into 4 Modules in CS Professional Programme

Module I
Company Secretarial Practice
Drafting, Appearances and Pleadings

Module II
Financial, Treasury and Forex Management
Corporate Restructuring & Insolvency

Module III
Strategic Management, Alliances and International Trade
Advanced Tax Laws and Practice

Module IV
Due Diligence and Corporate Compliance Management
Governance, Business Ethics and Sustainability