Hello,
For doing a diploma in engineering after your class 12, you must have secured atleast 50% marks in aggregate in your class 12 and passed from science stream.
Diploma in engineering is a three years course. Once you do your diploma, you can directly go for BTech directly into the second year through lateral entry.

Some of the top diploma in engineering colleges in India are

• LPU Jalandhar
• Jamia Milia Islamia Delhi
• Dr Vishwanath Karad MIT World Peace University, Pune
• Veermata Jijabai Technological Institute, Mumbai
• Techno India University Kolkata.

Please visit this link for more such diploma colleges in India
Hope it helps

Hello there,
Basically, any applicant who completed 10th standard are eligible to apply for Diploma course. There is no specific age limit to apply for Diploma course. Since, you have completed 12th standard so, after completing 12th standard you're eligible to apply for Diploma or Polytechnic Engineering course. But, being a 12th student, I would personally suggest you to apply for Bachelor degree course in Engineering instead of polytechnic engineering course/ Diploma course. Because bachelor courses will get better career option. You may apply for the Engineering courses like,B.E or B.Tech course after completing 12th standard in science stream.
And for polytechnic you have to appear for jexpo examination.
Here's a link which will give you a detail of jexpo.
https://engineering.careers360.com/articles/jexpo
Still for your further analysis I'm providing a link about Diploma courses,
https://engineering.careers360.com/articles/polytechnic

Hope this answer helps you.
Good luck!!
Thank you.