How do I make a sphere spin forever?

Here's a simple and easy-to understand one:

sphere = script.Parent --This means that you have to have the script in the sphere part
a = 0
    repeat
        sphere.Rotation = Vector3.new( 0, a, 0) --The second value of vector3 is a,
        wait(.01) -- we wait .01 seconds,
        a = a+1 --a's value increases
    until pigs == 1 --Just make sure you never have pigs' value to 1, and it'll spin forever

Hope this helped!

The following script will cause a ball to spin indefinitely. Modify the xDelta, yDelta and zDelta values to adjust the amount of spin on those axes. If you do not want any spin on the xAxis, for instance, set xDelta to zero.

local xDelta = 0.1
local yDelta = 0.1
local zDelta = 0.1
local ball = game.workspace.Ball
local origCFrame = ball.CFrame
local index = 1
while true do
    ball.CFrame = origCFrame  * CFrame.Angles(xDelta * index, yDelta * index, zDelta * index)
    index = index + 1
    wait(0.05)
end

By manipulating the Z axis.

local spinner = script.Parent --Put this script inside a part.

while wait(.03) do

spinner.CFrame = spinner.CFrame * CFrame.Angles(0, 0, .1)

end