Using input from html boxes in javascript

Using input from html boxes in javascript

Currently, I am trying to learn js. At this moment, I can make some simple
functional scripts. But whenever I want user input, I still have to us a
prompt, which can become quite annoying. Therefore, I want to use HTML
boxes for user input.
I put the box in HTML like this:
<FORM>
<INPUT type="button" value="Fill Me In" name="box1">
</FORM>
But how do I call the input in javascript then?
Thanks in advance.