Hi LVM Reddy, onsite opportunity first of all requires two main criterias,

1) Experience in the field of work

2) Knowledge in the field of work

One needs to develop good knowledge of the field or stream of work in order to prove its worthiness. Also one fresher or someone doesnt normally get chances to go onsite. Companies generally open up the onsite opportunity for all. But one can move smartly if he has a minimum of 3-4 years experience and very good practical Knowledge to ace a screening test. You also need to maintain a good rapo with your managers as they are the ones who could suggest you options and pave way for quicker chances to go in.