There's a few ways of doing it:

approach one:

local myString = "Hello World"
local characters = {}

--putting string in table by looping through each position in the string
--and getting the substring at that position (the character at that position
for i = 1, #myString do 
    table.insert(characters, myString:sub(i,i))
end

--print out the characters array to show that we put the string in the table:
for _, character in pairs(characters)do
    print(character)
end

approach two:

--use string.split("") to split up every character and put it into a table
local myString = "Hello World"
local characters = myString:split("")

--print out the characters array to show that we put the string in the table:
for _, character in pairs(characters)do
    print(character)
end

Both ways work, but I prefer string.split(myString,"") (approach two) since it's faster to write out.

Both have same speeds really.