The document - object model
Values can be either strings of words or numbers. if you enter a number into a text box it will automatically be a word. To convert it to a number, use the parseInt(the value of your input) method. Here's an examplet:
howMany = document.forms['numbers'].elements['someBox'].value;
plusTwo = parseInt(howMany) +2;
You probably have no idea what that does :-P Just look for the parseInt(howMany) ...
I hope that's enough to get you started... at least you will be able to understand the terms most used in more technical tutorials. I'll be adding a few examples of scriplets that work, and hopefully you will be able to learn from example.