I forgot my macbook password

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

When the menubar appears select Terminal from the Utilities menu. Enter resetpassword at the prompt and press RETURN. Follow instructions in the dialog window that will appear.

After resetting the login password, you have to do something to your keychain. Presumably you don't have the keychain prompt upon initial login anymore, so go into Applications > Utilities and open Keychain Access. Then reset your keychain in the Keychain Access > Preferences > General window. As a result, your saved passwords will be gone and you will receive a few prompts for various applications that are trying to access them. Remember to select "remember this password in my keychain" when entering the passwords into the prompts.

For more information on resetting original admin passwords, see http://support.apple.com/kb/HT1274.