When I have to select New promotion request in Windchill?

Can you explain a little more of what you need?  You can use it after you select a file at any time but it depends on what you are trying to do.

You use a promotion request to get files from one state to another instead of manually setting the state.  One advantage is it allows a note to be added to why you are promoting the file.

Help Center has pretty well documented material on Promotion Processes, Of course the default ones. It is worth a read. - http://support.ptc.com/cs/help/windchill_hc/wc110_hc/index.jspx?id=LCChp_PromotionProcess&action=show

I think Praveen Minumula ask : When I must create New promotion request in Windchill?

In Windchill many wais change from one state to another (Set State, Promoting etc..), but only Promoting can change state for specific baseline of objects (WTPart, EPMDocument, WTDocument etc.)