I have an HTML form as follow:
The feature you're looking for is called a "placeholder". (if nothing else, just knowing this term will help you search for more info in Google)
In modern browsers which support HTML5, this is a built-in feature which you can use very easily, as follows:
However, this method only works with up-to-date browsers which support this feature.
Older browsers will need to have some Javascript code to do it. Fortunately, there are a number of scripts you can use, including some written as JQuery plug-ins. The ones I'd recommend are those which tie into the placeholder
attribute on the input field, so that you can support it natively in the browsers which have this feature and fall-back to Javascript for those that don't.
Try this one: http://www.hagenburger.net/BLOG/HTML5-Input-Placeholder-Fix-With-jQuery.html