Go to Accounts Page (use the drop down box on the top right corner of your Google.com page). You will need to re-enter your password for addtional security.
In the accounts page click on the link, which might read as "Close entire account and delete all services and info associated with it"
Read all the information that say, it would DELETE ALL THE GOOGLE DATA, including your Picasa, Google+, GMail and other important data.
Below webpage URL shortcut might directly take you to google delect account page.