There are certain segments in CDS- Indian Military Academy, Indian Naval Academy, Air Force Academy, Officers' Training Academy - Men and Women. And the eligibility criteria is different for different streams. For IMA, INA and OTA men and women, the minimum educational qualification required is Degree in any discipline from a recognized university. For Air Force, a degree from a recognized university of degree in Bachelor of engineering is required. And also the candidate must have studied maths and physics at intermediate level. The age limit is also different for different streams. The candidates who are interested in IMA, INA, their age must be within 19 to 24 years. For Air Force, the age limit is 20 to 24 years and for candidates interested in OTA, their age must be within 19 to 25 years. In order to appear for CDS, you need to qualify your under graduation first. So if you have studied Humanities in +2, you need to complete your bachelor's degree. There is no barrier in course programme. It can be any discipline. It's only different for AFA.
http://competition.careers360.com/articles/cds-eligibility-criteria