how can i change my password when i open my mac?

If you don't remember your password, you have to reset the password. If you are using Mac OS X Leopard or Snow Leopard, see "Resetting the original administrator account password" > http://support.apple.com/kb/HT1274

If you are using OS X Lion or Mountain Lion > http://discussions.apple.com/docs/DOC-4101

Change password?

http://support.apple.com/kb/PH10746

Reset Password?

OS X 10.7 Lion /10.8 MountainLion

Follow the instructions in the first and the third boxes.

http://pondini.org/OSX/Password.html

Note

Keychain

http://support.apple.com/kb/TS1544

Best.