How to make forefield appear on you when you press "f"?

You create a ForceField using the new method of Instance, which may look familiar to you:

Instance.new("ForceField", game.Players.LocalPlayer.Character) --first argument is the 'classname', second (which is optional) is where to parent the created Object.

By default, this is named 'ForceField', so the inside of that if statement could look like this:

if game.Players.LocalPlayer.Character:FindFirstChild("ForceField") then
    game.Players.LocalPlayer.Character.ForceField:Destroy()
else
    Instance.new("ForceField", game.Players.LocalPlayer.Character)
end

Use booleans so the script knows the state of the forcefield (on or off).

local script = script.Parent
local Shield = false
FF = Instance.new("ForceField")

Then code this something along the lines like this, using if and else.

if string.lower(key) == "f" then
local FF2 = FF:clone()
    if Shied == false then
        Shield = true
        local FF2 = FF:clone()
        FF2.Parent = game.Players.LocalPlayer.Character
    elseif Shield == true then
        if game.Players.LocalPlayer.Character:findFirstChild("ForceField") == nil then return end
        Shield = false
        FF2:Destroy()
end

^^This should be in place of the if statement that's in your anonymous function.

I don't have any idea on how to make it disappear when hit again, but to give them one when they hit it once, do this.

local script = script.Parent

game.Players.LocalPlayer:GetMouse().KeyDown:connect(function(key)
    if string.lower(key) == "f" then
        local ff = Instance.new("ForceField") --Creates the ForceField
        ff.Parent = game.Players.LocalPlayer.Character --Assigns the parent to the ForceField
    end
end)