In Vanilla you cannot see other players' or NPCs' health, only your own. There are addons such as mobinfo2 that show health, but they do this by calculating it from the damage you deal, i.e. you have to damage a mob to see the health and even then it won't be displayed on the bars.
To install addon find the location of your VANILLA World of Warcraft folder, it is likely called something like "World of Warcraft Classic". Go into the interface and create an folder called "AddOns". You will place you EXTRACTED addons in this folder.
Example:
C:\Program Files (x86)\World of Warcraft Classic\Interface\AddOns
Here are some threads with addons, also you can usually find an addon you need by doing a Google search, just make sure you type all the details correctly.
viewtopic.php?f=32&t=605
viewtopic.php?f=32&t=10090
