how can I make a Screen GUI countdown? (Read desc. to find out what I talking about)

To accomplish this, you can use the For loop! :D

What the For loop does is (if it isn't the generic for) iterate a number of times (or count-up as I should say XP ).

Setting up a For loop is actually fairly easy; this's how you set one up:

for i = STARTVALUE, ENDVALUE, INCREASEMENT do
	-- Pretty self-explanatory names XD
end

And wa-la! You just set up a For loop! :D (Er... I did, actually. ;-; )

Now for the countdown part, the main reason we're doing all of this! >>:O All you need to do is set up the values in the for loop; personally, for countdowns, I like to have the For loop increase & have it subtract the end number; this is how it'd look like:

for i = 1, 20, 1 do -- The start value is 1 (where to begin), the end value is 20 (where to stop), and how much the value increases
	print(20 - i) -- Subtracts 20 by the current iterated number (i, essentially lol )
	wait(1) -- To wait before going onto the next iteration/ looping again; the wait function yields the code for a fixed amount of time, the time being in seconds. :P
end

And, wa-la (again)! That's a countdown code-script-thing... Idk T-T, but it will countdown! (Read the below for a recap on the For loop/ how you set it up.)

Now, to recap/ touch on, the for loop has 3 arguments: the first two are a MUST, while the third one is... meh lol; the first argument, STARTVALUE is the beginning value/ where the iteration begins, the second argument, ENDVALUE is the end value/ where to end, and the third argument, which is, again, optional lol, is how much the iteration increases: here's an example of what I mean:

for i = 0, 50, 2 do
	print(i) -- 0, 2, 4, 6, ...
end

And that's pretty much the ups-and-downs on how to use a For loop. :P

Hope this helped you in any way! :D