Hey Deepak,

There are many internships available in every company. As many of my friends did is they sent their CV with a cover letter of why they need the internship to the HR and they get the reply from HR within 10 days. These are the normal practice we do to get an internship. 

So for your branch, you can go for 

Indian Oil or (IOCL)

ONGC (Oil and Natural Gas Corporation)

These are the best training opportunities where you can apply. To get into any one of these you need to have a reference as because these are govt. most valuable industries. 

As far the Date to apply, It's done during the Summer Vacation that is after the semester is over.