IvyTies recently hosted a live webinar on life at Stanford University, in which Dr. Krishna Savani talked about his journey to Stanford University.

Now, to answer your questions:

I am from commerce background. Which department should be suitable for me?

This is something that you should figure out. Of course, other people can give you suggestions and opinion, but what ultimately matters is you - what you want to do? What interests you the most. Universities like Stanford will help you to figure it out on your own.

Are the teachers friendly enough to the foreign students like me?

Well, don't even worry about that :) There is no discrimination on any grounds

I'm a Muslim, will that be any problem while studying there?

Universities like Stanford has a substantial amount of international Muslim students and they do respect the values and diversity of each religion. 

What is the average cost of studying there?

IvyTies already have this information, just go to the Stanford University page on IvyTies and scroll to the section "Cost of attendance".