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Autodesk's Revit MEP families guide available for free to download off their website. They have a light fitting typical example in there and also give guidance on principles of content creation. Also only ever use non hosted templates or face based templates - not ceiling/wall/floor/roof based templates.
If you use theAutodesk wiki site and search for Families Guide, there should be an entry which links you to the downloads. The wiki is better than the help as the search facility works better and people can/do contribute to it