Literally hundreds of JS tutorials out there, but if you know AS3 you'll find that most of them won't take you far enough. As to dynamically assigning a value to a text field, HTML has text boxes and other elements that you can use. If I'm not wanting user input into the field though, the simplest thing is to use a <span> element and set the innerHTML property to the value:
Important thing to remember is that you can't address an element until it has been rendered, so if you put the script in the <head> section you'll need to wait until the document loads before allowing it to execute.
document.getElementById("spanToReplace").innerHTML = "This span now has a value";
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