I want to make a dialogue box that presents text like an RPG game but I can't figure out how to make it work. Any help?

this is the best i could get and it won't even work
(my text box's variable is "speach" without the quotes)
Actionscript Code:
speach.length = 0
speach= "This is the first dialogue".
onEnterFrame = function(){
speach.length++;
}

I don't understand why that won't work. Any help?