How do I make these blocks do Damage?

I made a function where when you press f 8 red bullets are shot from your character(with fire on them) I want them to do damage but I have no idea how, can anyone help? Also, this only works in Studio, not on a server. When I try to play it on a server is says " Players.Player1.Backpack.LocalScript:5: attempt to index field 'Character' (a nil value)"

Player = game.Players.LocalPlayer

Char = Player.Character

Torso = Player.Character.Torso

Mouse = Player:GetMouse()

Attack = false

Mouse.KeyDown:connect(function(key)
	local key  = key:lower()
	if key == "f" and (not Attack) then
		Attack = true
	   for i = 0, 8, 1 do
	        local Ball = Instance.new("Part")
	        Ball.Name = "Bullets"
            Ball.Parent = game.Workspace
            Ball.Shape = "Ball"
            Ball.Size = Vector3.new(1,1,1)
            Ball.Transparency = .4
            Ball.TopSurface = "Smooth"
            Ball.BottomSurface = "Smooth"
            Ball.BrickColor = BrickColor.new("Really red")
            Ball.CFrame = Torso.CFrame * CFrame.new(0,0.5, -5)
            Ball.CanCollide = false
            local Fire = Instance.new("Fire")
            Fire.Parent = Ball
            local Bv = Instance.new("BodyVelocity")
            Bv.Parent = Ball
            Bv.maxForce = Vector3.new(math.huge,math.huge,math.huge)
            Bv.velocity = Torso.CFrame.lookVector * 100   
            wait(.05)
	   end
	   wait(4)
       for _, v in pairs(game.Workspace:GetChildren()) do
         if v.Name == "Bullets" then
          v:Destroy()
          end
       end
    wait(1)
    Attack = false
	end
end)

EDIT: Sorry about the link. That was for instakill, I thought it would be TakeDamage(100). Anyways, you would do humanoid:TakeDamage(damage) to hurt players. I used pseudocode for this, so you need to define humanoid and you can replace damage with a number, or create a variable named "damage" and set its value to a number.

First off, it doesn't work in a server because the script will load before the character does.

Add this before defining the character

repeat
    wait() until Player.Character

Second, you need to add a .Touched event. As YumTaste mentioned, you can refer to my blog post about this similar topic, but when you get the the BreakJoints(), replace it with the following:

local hum = plr.Character:FindFirstChild('Humanoid')
if hum then
    hum:TakeDamage(20)
end

You can change the 20 to whatever you want the damage to be.

Edit: Here

Player = game.Players.LocalPlayer;
Char = Player.Character;
Torso = Player.Character.Torso;
Mouse = Player:GetMouse();
Attack = false;

Mouse.KeyDown:connect(function(key)
	local key  = key:lower()
	if key == "f" and (not Attack) then
		Attack = true
		local bullets = { }
		for i = 0, 8, 1 do
		local Ball = Instance.new("Part")
		Ball.Name = "Bullets"
		Ball.Parent = game.Workspace
		Ball.Shape = "Ball"
		Ball.Size = Vector3.new(1,1,1)
		Ball.Transparency = .4
		Ball.TopSurface = "Smooth"
		Ball.BottomSurface = "Smooth"
		Ball.BrickColor = BrickColor.new("Really red")
		Ball.CFrame = Torso.CFrame * CFrame.new(0,0.5, -5)
		Ball.CanCollide = false
		local Fire = Instance.new("Fire")
		Fire.Parent = Ball
		local Bv = Instance.new("BodyVelocity")
		Bv.Parent = Ball
		Bv.maxForce = Vector3.new(math.huge,math.huge,math.huge)
		Bv.velocity = Torso.CFrame.lookVector * 100   
		wait(.05)
		table.insert(bullets, ball)
		ball.Touched:connect(function(part)
			if part then
			local plr = game.Players:GetPlayerFromCharacter(part.Parent)
			if plr then
				local hum = plr.Character:FindFirstChild('Humanoid')
				if hum then
					hum:TakeDamage(20)
					ball:Remove()
				end
			end
		end)
	end
	wait(4)
	for _, v in pairs(bullets) do
		v:Destroy()
	end
	wait(1)
	Attack = false
	end
end)

That should work. Let me know if it doesn't.