Hi,
It differs from hostel to hostel. For (http://www.jubileehall.in/docs/HB%202018-19.pdf) JubileeHall Hostel one needs to apply online. Admission to the Gwyer Hall will be givne on the basis of merit list provided by the concerned departments/faculty of the University which will be be drawn up on the basis of the merit lists of different departments and faculties, with seats distributed according to the percentage of their total allocation in Gwyer Hall.

Complete list of DU hostels is available at http://www.du.ac.in/du/index.php?page=hostels . You can follow their link to gather more information.