I forgot the Password for WPA/WPA2 Personal where can I get it I'am trying to set up a wireless connection to my HP OfficeJet printer. For this I need to enter the password to my WLAN which I forgot.

How and where can I retrieve it?
Open the Keychain Access app (located in the Utilities folder, within the Applications folder).
In the search box (upper right corner), type in the name of your Wi-Fi network.
Double click the corresponding entry in the main panel. If there is more than one, look for the most recent one with a type matching "AirPort network password".
Click the checkbox labelled Show password, and enter your user account password when prompted. The password will be displayed in the text box, where you can copy it as needed.