Hii,Yes,sure yopu can join diploma in mechanical enhineering.And the duration of course is 3 year and theminimum eligibility criteria is passed out 10+2  in Science stream and from an recognized educational Board.

Here,the top institution who offered this course:

-ITM Vocational University,Vadodra

-Wisdom School of Management,Ahmedabad

-International Institute of Mnagement and Technical,Ghazaiabad

- Roorkee College of Engineering,Roorkee

-School of Engineering and Technology,Jaipur

-Lovely Professional University,Jalandhar

Also there are some entrance exam also held to enroll these course.Hope,this information helpful for you,Goodluck for your future.Hope,you do ur best.