How do I make a randomiser that activates a script?

You can use the function math.random(min, max) to generate a number, where min is the minimum number, and max is the maximum number which the function can return any number between (inclusively)

i.e. math.random(1, 3) can return the numbers: 1, 2, or 3

An example of this with your scenario would be:

local min = 5 * 60 -- To determine time of (X = 5) minutes in seconds
local rain = script.Parent:WaitForChild("RainScript")

while true do -- Whatever loop you want to use
    wait(min)
    local random = math.random(0, 20)
    if random < 10 then
        if rain.Disabled == true then
            rain.Disabled = false
        else
            rain.Disabled = true
        end
    end
end

local number = 0
local raining = false

while true do
    wait(300)
    number = math.random(1,20)
end

if number >10 then
    raining = true
else
    raining = false
end

I'm not entirely sure if this is what you wanted but every 5 minutes it will choose a random number between 1 and 20. If the number is less than ten then the raining variable will be false, and if it is greater than ten it will be true.