Dear Anjali, CDA/CDS stands for Combined Defence Services. for working with CDA, you should appear for CDS Entrance Examination conducted by UPSC. For starters, you need to be between 20-24 years for being eligible. Candidates who are in their final year of graduation or have completed graduation can appear for this Entrance Examination. The exam was already conducted on February 3rd for this year. You may have to try next year.