selection of more then one value in selection input box

sir, i am not able to select more than one value each seperated by comma in a input selection box.
ex:- if there is a selection dropdown box with options of cities like patna,gurgaon,kanpur,asam,manipur,banaras,then the result should be like in the drop-down box if i want to select patna and kanpur together,then the patna and kanpur shuld apper like patna,kanpur.