I'm new to scripting, this is my first time using char and nothing works with it just wanna know?

script.Parent.Activated:Connect(function()
    if canForm == true then
    local form = Instance.new("ParticleEmitter")
    form.Texture = "http://www.roblox.com/asset/?id=709137722"
    form.Size = 2
    form.Color = Color3.new(116,81,58)
    wait(0.6)
    canForm = false
    form:Destroy()
    char:FindFirstChild("RightArm").Material = "Corroded Metal"
    char:FindFirstChild("Shirt"):Destroy()
    end
end)

###This is because in the R6 rig of the character, the right arm is called Right Arm, not RightArm. The material is Enum.Material.CorrodedMetal, not "Corroded Metal" with a space. There were a few things you did wrong as well, like using Color3.new without dividing each number by 255.

script.Parent.Activated:Connect(function()
    if canForm then -- no need for == true'
        local form = Instance.new("ParticleEmitter")
        form.Texture = "http://www.roblox.com/asset/?id=709137722"
        form.Size = 2
        form.Color = Color3.new(116/255,81/255,58/255)
        form.Parent = script.Parent.Handle
            -- Assuming this is a tool, I put it in the handle 
        wait(0.6)
        canForm = false
        form:Destroy()
        local ra = char:FindFirstChild("Right Arm")
        
        if ra then
            ra.Material = Enum.Material.CorrodedMetal
        end
    
        local shirt = char:FindFirstChild("Shirt")
        
        if shirt then 
            shirt:Destroy()
        end
    end
end)

###You should also be checking if objects exists via if statements, as code won't work if the object doesn't exist.