How do i make a countdown/up timer?

We don't make script at request on here, we ask questions about premade scripts. To do this, simply select an amount of seconds, let's say 100 for this example.

local Time = 100 -- What the timer should start at
local Goal = 0 -- What the goal is for the timer
local Interval = -1 -- How much it will decrease the timer every second. Change this to 1 if you want to count up.

-- Determine whether the loop counts up or down and start a loop
if Goal > Time then
    while Time < Goal do
        local Minutes = math.floor(Time / 60) -- Since time is amount of seconds the timer is on, and 1 minutes is 60 seconds we divide Time by 60. To avoid decimals we floor that number.
        local Seconds = Time - Minutes * 60 -- We see how many seconds are left after we take out the minutes.

        print(Minutes..':'..Seconds) -- Print this timer
        wait(1) -- Waits 1 second before the loop runs again
        Time = Time + Interval
    end
else
    while Time > Goal do
        local Minutes = math.floor(Time / 60) -- Since time is amount of seconds the timer is on, and 1 minutes is 60 seconds we divide Time by 60. To avoid decimals we floor that number.
        local Seconds = Time - Minutes * 60 -- We see how many seconds are left after we take out the minutes.

        print(Minutes..':'..Seconds) -- Print this timer
        wait(1) -- Waits 1 second before the loop runs again
        Time = Time + Interval
    end
end

You still need to add a way to display the timer yourself. Hope this helps. Keep what I said at the beginning of my post in mind!