The onboarding queue is used to see what outstanding work you have left to do for onboardings. Go the "Human Resources" Section and click on "Onboarding Queue".
You can search for specific onboardings or scroll to the bottom of the page to see all onboardings for which you have items to complete.
To view a questionnaire, click on the view button in the Actions column.
This will bring you to the Questionnaire to fill out.
Use this helpdesk article for more information on how to complete a questionnaire.