What should I do after 12th to join in Income tax department?

In the order to become Income Tax Officer, you need to appear in the Civil Services Exam conducted by the Union Public Services Commission (UPSC)

The minimum requirement to apply for the Civil Services Exam is graduation, so you should wait till then the completion for your graduation, you are not eligible to apply for it just after 12th.

Here I am giving you the details of this examination:

Eligibility:
The applicant should have completed graduation with good academic record from any recognized university of India.

Age:
Age between 18 to 28 years, age relaxation as per norms, as under
For SC/ST 5 years
For OBC 3 Years

Physical Standards (Minimum)
Height :157.5cm
Chest :81cm ,fully expanded with a minimum expansion of 5 cm
Physical Test:
Walking:1600 meters in 15 minutes
Cycling:8 Km in 30 minutes