I just passed 12th so Which is better among Diploma in mechanical or BCA for good job ?

if you are interested in computers then go for BCA you good scope and jobs in it otherwise Diploma is also good you easily get 15-20000 jobs

Diploma is not equal to degree. Eligibility of Poly technic Diploma is 10th Std only. BCA is professional / applied science graduation IT / Computing Field. After BCA, you can get job opportunities as Junior Application Developer, Application Developer, Software Tester / Test Engineer, System Adm