If I would find myself in 272 BC I would ...

.. learn languages(Greek and Latin most likely) first few years in some small town. Then probably would move to Syracuse and warn Archimedes to get out of the city before the Romans come. Once that done I would head to Byzantion and try to take control. Would probably make some reforms and introduce new technologies(not gunpowder). If I can keep mayor powers and barbarian hordes away from the city then I guess after ten-twenty years I would have enough power to start my world conquest!