Input Text Box - Placeholder Text

I have a simple input box where people can type a message in it and send it off in an email. However, I want people to know what the box is for and that you can type in it. A simple “Type your message here” line would do the trick, but if I have that text pre-typed into the box, the user has to highlight it and delete it before they start typing. How to I make it so that as soon as the user clicks the text box, the placeholder text disappears? I’d really appreciate any help… thanks!