I've just logged into my account online how do I access last year's (W-2) or tax return?

Your W-2 isn't stored in TurboTax, but you can access your tax return if you are using the same account you did last year.   When you sign in,  click on Tax Home in the left side panel.   Then scroll all the way down to the bottom to find your previously filed returns.    Expand the section titled Your tax returns and documents.   

If you need this year's W-2, you have to get it from your employer; TurboTax doesn't have access to your W-2s.   You may be able to import it in the Wages and Income section, but you will need the W-2 control number....which you can get from your employer.