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 do a search to find specific onboardings or just scroll down to the bottom of the page to see all onboardings you have items to complete for. 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 fill out a questionnaire.