Learn how to set screen lock on Nexus 7. Depending how you use your phone, you may want to set some form of automatic screen lock to prevent access.
Unfortunately there is no way to recover your password. You will need to restore your Kindle Fire back to the factory settings. When you enter a wrong password four times in a row youll be directed to restore your device. This process will delete everything on the Kindle Fire and it will need to be registered again so you can download content from your Amazon account.