How to make a text appear gradually?

What I want to do is make a text appear letter by letter, like a dialogue in a classic RPG, but I am kind of clueless of how to do it.

My only idea is this: 1. Use string.length to count the number of characters of the string, and save it on a variable (let's call it char_cap). 2. In a loop, take out the number of characters of the text indicated by char_cap, then take out 1 to char_cap, and repeat. This should make the text "grow" one character every frame. 3. When char_cap reaches 0, the loop ends.

But I don't know how to take out a certain number of characters from a string like it's described in step 2. Maybe I'm just going the wrong way, I'm pretty noobie at this. Any suggestions?