✓ Accepted answer
Firstly, you cannot use the word "string" as a variable, because it is a Lua keyword.
Aside from that, what you need to do is increment through all the letters of your string using a for loop. Here's how you would do it:
local word = "Hello"
local length = string.len(word)
for i = 1, length do
print(string.sub(word, 1, i))
wait(0.5)
end
Here's a link to the ROBLOX Wiki on for loops in case you're confused about what just I did. I highly recommend you read it if you don't understand how a for loop works. They are very important and useful for programmers. :)
Hope this helps! if it does, don't forget to click "Accept Answer" and +1 my Rep!