Easiest way is to just hit forget my password and have an email sent where you can change it.

It is recommended that it be 8 characters long, one capital letter, one lower case letter, one number and above all else, something you will remember.

If you get the following error message:

"that account is managed by an external authentication provider. Please visit their site to reset the password”. 

What has happened is the account was most likely signed in with the google account associated with it.