Help Formatting String into URL

Hi, Can someone help me write a short code snippet to format a string to add: http://www <http://www/> -- if the string is "google.com" and doesn't have http://www <http://www/> in front of it http:// <http://www/> -- if the string is "www.google.com" and doesn't have http:// in front of it .if you know of a better way to securely check/format a url like this please let me know. Thanks, Ryan