I have recently completed Btech in Electronics and Communication. Now I have to work in an IT company, what is the most important for me to know- C language, C++, Java or anything else ? Which course should I do.. before going into that company ?

What type of work does the IT company do. Did you ask them what additional skills you may need ? They would know what was most important,and they are the ones you need to ask.

"IT Company" is a vague term. For example Microsoft is an IT Company, but there are many different jobs at Microsoft. It all depends on the type of job and career you want.