Please suggest where i must work and some other doubts............?

You are very unlikely to be hired as a permanent scientist (civil servant) at NASA - to hire a foreigner for that job, they must convince the hiring committee that no one in the US could do that job. You'd need to be a citizen to have decent shot at it. However, it's a bit easier (although still very competitive) to get into NASA as a postdoc for 2-3 years as a non-citizen.