How do I implement string.sub into my script?

local ClickDetector = script.Parent.ClickDetector     --Makes the GUIs visible, then does the text.
ClickDetector.MouseClick:connect(function(User)
local player = User    
local dialogue = player.PlayerGui.ScreenGui1.DialogueBox
local close = player.PlayerGui.ScreenGui1.CloseButton
local name = player.PlayerGui.ScreenGui1.NameBox
local option1 = player.PlayerGui.ScreenGui1.Option1
local option2 = player.PlayerGui.ScreenGui1.Option2
name.Visible = true
option1.Visible = true
option2.Visible = true
close.Visible = true
dialogue.Visible = true
name.Text = "Cassie"
option1.Text = "Sure!"
option2.Text = "No, thanks."
dialogue.Text = string.sub("Hello.", 1, 6)
end)

This is everything I have so far. It is for my rpg game where I am making my own dialogue gui. I want the text to appear letter by letter, and I think that in order to make that work, I need to use string.sub for maximum efficiency. My problem is, I don't know how to efficiently change the value of the second number '6' or any other amount by adding 1. Of course, it'd start as only having 1, 1 then I need it to add 1 to the second number until everything in the string is on screen, then it'd stop until activated again when an option is pressed (which is in a different script) So, please help?

So to have text be "typed out" as you request, you'd need to show the text you want in different phases, from when the first character is typed, to the last character. In order to do this, you will need to find the total amount of characters using:

string.len()

Now once you have the amount of text, you'd want to run a loop to type out the text, piece by piece. This is where string.sub() comes into play, it is used to read text at a designated starting point to a designated ending point. For example:

print(string.sub("Hi my name is Paul", 1, 5))

Would print - "Hi my", starting with the 1st character of H and ending with the 5th character of y. (This includes the spaces between texts)

The for loop uses both, string.sub() and string.len(). Now to bring this all together, you'd make something along the lines of...

local Message = ("Hello this is some kind of message I wish to display")
local TextBox = script.Parent:WaitForChild("TextBox") --- Let's just say this is inside a frame

for i=1, string.len(Message) do -- Remember, string.len() gets the amount of characters in the string!
	TextBox.Text = string.sub(Message,1,i)
	wait()
end --- The most important thing, if there isn't a wait, then the for loop runs instantaneously, so the visual effect doesn't happen. 

And ta-da, you've got yourself a typed out message. Now it's up to you to implement this into your own script. If you've got any other questions, feel free to hit me up.