Fliflo sound funny...
Yeah, both AVE and the night train (the Lusitania) have compulsory reservation. I suppose that there's an international counter at Barcelona Sants where they can help you. The other big station in Barcelona is Franca.
I'd recommend making the reservations as soon as you can since the night train is your only (sensible) way to get to Lisboa. If it'd be full already you could try the bus (follow the link I posted) or go with Sud Espresso Irun - Lisboa (use the travel planner and fill in "via Salamanca" or "via Miranda de Ebro").