If you have never lived in the US, you can probably still register to vote

Thirty-seven states and the District of Columbia extend voting rights to the children of their former residents who have never resided in the US. Unfortunately, thirteen US states do not extend voting rights to citizens who have never lived in the US. For the latest information concerning each state's position on the subject, please refer to the FVAP website (www.fvap.gov). ACA encourages US citizens living abroad to register and vote in the state where they last resided, or in the state in which their parents last resided if permitted by the law of that state.