DEAR Varsha,

as per your profile, you have completed only 12th class

In that case, you are not eligible to apply for this exam. 

Opt for the stream in Graduation in accordance with the stream in 10+2 and then you can apply for CDS Exam.

However you can make entry into the following defence services by cracking NDA Exam.

This is the common written exam to recruit the candidates into 
Army wing
Navy wing 
Air Force wing
Hope it may help you.......

Dear Varsha,
You will be eligible for CDS, Since you are a girl you can take part in this exam but opt only for OTA provided you fulfill all other parameters set for this. I am providing you the link where you can get detailed information about CDS I 2018.

I hope it helps!