You didn't mentioned the state in which you reside. Now to gain a Diploma in any field you have to sit for the entrance test of polytechnic. So I would like you to search the name of the polytechnic exam of your state. Having a good rank in it can give you a admission to Diploma in Computer Science & engineering course which is of 3 years duration.

For West Bengal, the name of the examination is JEXPO.

I hope my answer helps you. All the very best for your future endeavors!