I want to get a cool down on my weapons. But how?

You can make a variable on the top and it will refer to a number. Then, add wait(thevariable) before canDamage = false. If you are confused, this is what I mean.

cooldown = 3   --You make this at the top so you can adjust it more convenient

local tool = script.Parent 
local canDamage = false
local humanoid = otherPart.Parent:FindFirstChild("Humanoid")

if not humanoid then 
    return 
end

local function onTouch(otherPart)
    if humanoid.Parent ~= tool.Parent and canDamage then   --You check if the player can attack here
        canDamage = false   --We make the player cannot attack once they activate the tool.
        humanoid:TakeDamage(25)
        wait(cooldown)   --Then, wait the 3 seconds then we make the player can attack again.
        canDamage = true   --After that, we make the player can attack again.
    else
        return
    end
end

local function slash() 
    local str = Instance.new("StringValue") 
    str.Name = "toolanim"
    str.Value = "Slash" 
    str.Parent = tool 
    canDamage = true
end

tool.Activated:Connect(slash) 
tool.Handle.Touched:Connect(onTouch)