Astronaut,
to make your url address clickable (so person may actually point, click on it and his browser will open a window with a site this link point to) you need to format your email at HTML. But I have question to you - what email program do you use? How do you send regular text emails?
The sample of your email at HTML is the following:
<html>
<body>
Your email text......
<a href="http://yoururladdress.com">
Yoururladdress.com</a>
again text...
</body>
</html>