I'm very new to the photography and don't have much knowledge. I have a Nikon L820 and want to add a hood and filter to it, however I don't know if it can be added directly, or what's the 'mm' rating of the lens thread might be.
the camera that you own, the Nikon L820 is a bridge /semi-pro camera. and there is no lens thread on the edge of your lens. so you can not fix an filter or a hood on your camera's lens (very unfortunate).
however you can make your own lens hood and fix it :p ,
- Paper cup / plastic cup
measure the diameter of your lens
Get a Paper cup and make a hole at the base of the cup with the same measurement as the diameter of your lens
Now combine the camera with the hood.fix on your lense and shoot away.
Behold the DIY lens hood.
enjoy mate. happy clicking .
A little searching shows that a company called Kiwifotos put together a hood and filter adapter kit. Should be just what you need; uses 62mm filters: http://www.ebay.com/itm/LENS-ADAPTER-FILTER-SET-NIKON-COOLPIX-L820-62MM-KIT-/330904438399