how to search people in search box in sharepoint 2013

That looks like the #SearchBox div. There should be a drop down the that allows you to select conversations, people, everything etc.. I believe you need to configure a search center. It's very easy as it is only a type of Site Collection.

http://technet.microsoft.com/en-us/library/hh582314(v=office.15).aspx

You then have the option to create a new 'Search People`page which does all the heavy query work for you. Here are the steps:

http://www.sharepointconfig.com/2013/05/how-to-create-a-simple-sharepoint-2013-people-directory/