Hi there I forgot my password on my mac then I can not install any thing on it how can I reset my password

OS X: Changing or resetting an account password

Forgot Your Account Password

For Snow Leopard and earlier

Mac OS X 10.6- If you forget your administrator password

For Lion/Mountain Lion

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.

Or see Reset a Mac OS X 10.7 Lion Password and

OS X Lion- Apple ID can be used to reset your user account password.