Tell him Rossio. That's smack in the center. If you're worried about getting ripped off, you can purchase a prepaid taxi voucher at the tourist desk right before you exit the arrivals area. It's 15-20€, I don't remember the exact price, but it's around what you'd pay on a non-fixed rate anyway. And while you're there get a free map there as well.

As far as not getting ripped off, make sure they turn on tariff 1 (it'll show on the meter as "Tarifa 1"), the starting price will either be 2,00€ or 2,50€.