I would strongly recommend NOT taking a car into central Lisbon as there are constantly increasing traffic control measures, that make life complicated, including one that bans any vehicle more than 20 years old from entering the central zone.
However, if one must, and if one is staying in the Restauradores, Rossio, Praça Figueira area, there are two underground public car parks.
One is situated under Praça Figueira itself and the other at the bottom of Avda Liberdade, by Restauradores. I know from experience that some better hotels are able to offer preferential parking rates in the latter.