I assume you mean your Apple ID password? Go here and put in your information: https://iforgot.apple.com
OK if you saved it in your keychaine you can get it back:
Open Keychain Access, found in /Applications/Utilities/Use the Search box in the upper right corner and type “iphone backup”
Double click the result, assuming something is found in KeychainCheck the box next to “Show password” and enter the Mac administrator password ***** reveal the lost password ***** with the encrypted iPhone backup
That is in Applications / Utilities on your Mac.