After diploma in mechanical engineering, you can opt for Btech in mechanical engineering, directly second year through lateral entry exam (LEET).

After earning graduation degree in mechanical you will eligible to apply for various private sector such as TATA Group, Kirloskar, Siemens, Geometric Ltd, Thermax etc. and government sector companies like Bharat Heavy Electricals Limited, Bhabha Atomic Research Center, NTPC, Indian Space Research Organization etc.

Also you can work for some duration and gain experience in this field and then go for further studies.

There are various short term courses like:

CAD/CAM training

CNC Training

AutoCad

Catia

Unigraphics and many more.

So accordingly you can choose.

Good Luck!