How to convert numbers to strings using JavaScript?

You can prepend the number with an empty string
var mystring = ""+myinteger;
or
var mystring = myinteger.toString();
You can specify a base for the conversion,
var myinteger = 14;
var mystring = myinteger.toString(16);

mystring will be "e".

How to change style on an element?

Between CSS and javascript is a weird symmetry. CSS style rules are layed on top of the DOM. The CSS property names like "font-weight" are transliterated into "myElement.style.fontWeight". The class of an element can be swapped out. For example:
document.getElementById("myText").style.color = "green";
document.getElementById("myText").style.fontSize = "20";
-or-
document.getElementById("myText").className = "regular";

How to have the status line update when the mouse goes over a link (The support of the status line is sporadic)?

<a href="javascript.shtml"
onmouseover="window.status='Hi There!';return true"
onmouseout="window.status='';return true">Look at the Status bar</a>

Look at the Status bar as your cursor goes over the link.

How to write messages to the screen without using "document.write()" ?

Changing the contents of an element is a much better solution. When the method showStatus is invoked it will change the content of the span.
...
function showStatus(message) {
var element = document.getElementById("mystatus");
element.textContent = message; //for Firefox
element.innerHTML = message; //for IE (why can't we all just get along?)
return true;
}
...
<span id="mystatus">Test. </span>

What is the difference between undefined value and null value?

1 Undefined value cannot be explicitly stated that is there is no keyword called undefined whereas null value has keyword called null

2 Typeof undefined variable or property returns undefined whereas typeof null value returns object