How do you make a NPC attack a person only when that npc is attacked?

This should work, but I haven't tested it at all. Next time don't just ask for a script though. :)

local NPC = game.Workspace.NPC -- The NPC's model, make sure to set it corretly.
lcoal Deb = fasle

function findNearestTorso(pos) -- This finds and returns the attacker (the neasrest torso)
    local list = game.Workspace:GetChildren()
    local torso = nil
    local dist = 100 -- Max distance to search for the person, can be low since the attacker should be relatively close(Ranged weapons are a another story)
    local temp = nil
    local human = nil
    local temp2 = nil
    for x = 1, #list do
        temp2 = list[x]
        if (temp2.className == "Model") and (temp2 ~= script.Parent) then
            temp = temp2:findFirstChild("HumanoidRootPart")
            human = temp2:findFirstChild("Humanoid")
            if (temp ~= nil) and (human ~= nil) and (human.Health > 0) then
                if (temp.Position - pos).magnitude < dist then
                    torso = temp
                    dist = (temp.Position - pos).magnitude
                end
            end
        end
    end
    return torso
end

function HealthChangedEvent()
    if Deb = true then -- Just so that the function doesn't activate every time the health changes, only when it is lower than 100
        return nil
    end
    while NPC.Humanoid.Health < 100 do
        Deb = true
        wait(1)
        local target = findNearestTorso(script.Parent.HumanoidRootPart.Position)
        if target ~= nil then
            NPC.Humanoid:MoveTo(target.Position, target) -- This actually makes the NPC move
        end
    end
    Deb = false
end

NPC.Humanoid.HealthChanged:connect(function(health)
    HealthChangedEvent() -- Every time the NPC's health changes, the follow function is called
end)