If you did not file your own tax return in 2015 then enter a 0 (zero) for the 2015 AGI.
Otherwise, if you filed a 2015 tax return and indicated on the tax reurn that you were being claimed as a dependent on someone else's return, then just use the AGI that was on the tax return You filed.
See this TurboTax support FAQ for a 2015 AGI - https://ttlc.intuit.com/questions/1900840-how-do-i-find-last-year-s-agi