You first need to go to iforgot.apple.com and click Forgot your Apple ID?. This will let you recover both ID and password.
Now, proceed with the installation.
You should also be able to install OS X without an Apple ID and set it up later.
The Keychain password typically is your user account password. The one you've been using on the Mac before to log in. Your Mac has prompted you for this password whenever you wanted to install software that came with an installation assistant, or in order to save credentials for websites in Safari, Mail account passwords, etc.
Those passwords are stored in the "login" Keychain, which is the default on every system. You can find out what the Keychain stored by opening Keychain Access.app, or examining the file ~/Library/Keychains/login.keychain
.