People who are claimed as dependents on someone else's return can -- and sometimes are required to -- file income tax returns, so it's not really relevant that you're claimed as a dependent.
That said, if you filed a return last year, you should have an AGI. Please see the TurboTax FAQ "How do I find last year’s AGI?" for guidance.
If you didn't file a return last year, please see the TurboTax FAQ "What do I put for my AGI if I didn't file last year?" below for guidance.