You made no mention of jQuery, or any other javascript framework, so I'm going to give you the pure Javascript solution.
Some boolean logic based upon the value of the box to set the font color. When the user clicks the input, if the value is equal to your default value, you erase the contents. If it's not, you do nothing. When the user leaves the box, if the values are empty, add your default text in again.
// Reference our element
var txtContent = document.getElementById("content");
// Set our default text
var defaultText = "Please enter a value.";
// Set default state of input
txtContent.value = defaultText;
txtContent.style.color = "#CCC";
// Apply onfocus logic
txtContent.onfocus = function() {
// If the current value is our default value
if (this.value == defaultText) {
// clear it and set the text color to black
this.value = "";
this.style.color = "#000";
}
}
// Apply onblur logic
txtContent.onblur = function() {
// If the current value is empty
if (this.value == "") {
// set it to our default value and lighten the color
this.value = defaultText;
this.style.color = "#CCC";
}
}