How one can enter in public sector like most of the jobs are offered by Pvt companies so how one can switch?

You can work as an consultant if you are keen about working with the government. Government relies much on the recommendations of these consultants. If you wanna switch to Government job then you can focus on SSC and Banking. PSUs like ONGC, NTPC etc., make recruitment for specialised posts , but not on regular basis. One has to be very promoted about the timings of the vacancies. Once you get into MBA course You will get to know alot more by yourself that's what exposure means. Hope this helps.