I was first called to interview by an employer to work in his group. They were mostly looking to use the internship to hire potential full time employees. They are otherwise unlikely to hire interns. If you are a PhD student, or perhaps not in your pre-final year of college, the best go is to apply for research and show that you have research credentials. Software expertise seems to be a requirement these days.

I sent application on official website, after 2 weeks I was contacted by mail. It was easy to organize an interview, but I asked if there was some topics which I could prepare for the interview, but I don't get an answer.