The eligibility for NDA is either appearing or passed 10+2. If you have already completed 10+2, you would be eligible. If it is not, you should be appearing in the final year of Diploma Course you are pursuing. You may refer to https://www.nda.nic.in/ for complete details.
I hope this information will be useful.
First of all you have to complete your diploma then only you can appear for NDA or when you are in final year appearing.
You can go through the link below