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 ,
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