To assume the role of customs office, you should get into the Indian  Revenue Service (IRS). IRS constitutes Income Tax and Customs and  Central Exercise. Customs deals with taxation of goods brought into the  country. Excise deals with taxation of goods manufactured within the  country. To get into IRS, you have to clear the Civil Services Examination. 
The Union Public Service Commission conducts the civil services examination for recruitment into services like Indian Foreign Service,  Indian Administrative Service, Indian Police Service, Indian AUDIT and  Accounts Service, Central Secretariat Service and Indian Revenue Service  (Customs and Central Excise), Group 'A'.

The eligibility criterion for the examination is graduation from a  recognized university. The candidate must be between 21-30 years of age.  
The Civil Services Examination comprises: 
(i)  Civil Service Aptitude Test (CSAT)
(ii)  Civil Services Main Examination 
(iii)   Personality Test
Civil Service Aptitude Test consists of two papers. Each paper  carries a maximum of 200 marks. Questions are objective type. The  syllabus includes subjects like Current Events of National and  International, HISTORY of India, Geography, Economic and Social  DEVELOPMENT, general issues on Environmental Ecology, General Science,  Logical Reasoning and Analytical Ability and English Language  Comprehension skills. 
Once you qualify the Civil Service Aptitude Test, you are eligible to take the Main Examination.
Civil Services Main Examination consists of nine papers. All the  papers are of descriptive type. These papers are intended to assess the  overall intellectual abilities and knowledge of the candidates. 
The Personality Test is an interview conducted to assess the  candidate's personality in terms of his intelligence, abilities, traits  and values.
Civil Services Examination is one of the toughest competitive  examinations in India. So, if you are aspiring for the Civil Services,  you have to be persistent and HARD working. You should update yourself  with developments across the world. You have to read magazines that can  help you improve your knowledge and aptitude. Practicing with model  question papers will help you PICK up speed.