Pass A Text Field To A Js Variable On Same Page?

A need has come up, where I have to pass a text box entry to a Javascript Variable on the same page. Once I have that variable, I will need to utilize it on various places in the page. Here are the segmants of code I am working with:

*** DECLARING THE VARIABLE "firstname" (this would appear near the top of the body) ***

<script type="text/javascript">
var firstname;
firstname="This is Where I would like the text box to place it's value";
</script>

[Code]....

I know that I can set the text value of a DIV or SPAN very easily, but that is not what I want. I really would like it in a Js Variable that I can utilize freely in the code, wherever I need it.

1: Is this at all possible

2: What code am I missing to make this work. Again - I am very, very limited with Js knowledge.