I recommend that you use jQuery to achieve this functionality without too much headaches.
There's lots of jQuery Form Validation plugins out there, just google a bit that you'll see.
But it's not hard to code your own.
The logic is kind of simple: When a input value changes, validate it's content and if it's not valid, show the warning message. Take care of hidding the warning message when the input became valid. It's also nice to differ a empty required field from a invalid content.