HI prabas

You can apply visa from USA. Just fill up the application form(http://www.visa4uk.fco.gov.uk/ApplyNow.aspx) and submit online and send your all the documents orginal passport, valid I 20( signature from international affairs), bank balance certificate, application form. You can get visa within 10 days if all the things are correct.

Let me know if you have any question

Good luck