The airport is quite busy, and there are usually long lines to take the taxi or the bus. The best way to get to the city center and the Universidade Nova de Lisboa from the airport is the metro (in the map you can see the Metro station AEROPORTO, just outside the Arrivals). You can take the red line until São Sebastião for the University; you need to change in São Sebastião and take the blue line to get to the City Center.
Otherwise, you can also take a taxi, but the best and quickest way is to go to the DEPARTURES. When you exit the main door of departures, you will find some taxis across the road waiting. There is no line and they will take you immediately. You can have an idea of where you should go here: