Let me know if you need more specific information than this.
Go into System Preferences and click on Accounts. Click the lock to make changes and then enter your password and click OK. Click the plus button to create a new account and select Administrator from the menu when entering the new account information. Once this is done log out of your account and into the new one you created. At that point you can go back into accounts and remove your account by highlighting it and clicking the minus button. You can choose the option here to delete the home folder, which will delete all of your files. But if you want to be more secure choose the "Do Not Change Home Folder" option. Then you can drag your user folder to the trash and do a Secure Empty Trash. This way the new owner could not recover any of your deleted files. Do that if you have any sensitive information on the computer and want the added security.