Subway from Lisbon to Lisbon (city)
Depending on where exactly you will dock you have different options to take a metro.
Metro from Santa Apolonia cruise terminal
If your cruise ship docks at the Santa Apolonia terminal, you will find metro stations Santa Apolonia and Terreiro do Paco within walking distance on your left and right side along the bank of Rio Tejo. Both stations are on the same blue line. Take a metro direction Amadore Este and get off at Restauradores (just 2 or 3 stops), right in Lisbon city center. From there you can also take take a train from Rossio train station if you want to get to Sintra.
Metro from da Rocha or Alcantara cruise terminal
If you dock at da Rocha Conde de Obidos or Alcantara, there is no metro station. Take a train at Alcantara-Mar train station direction Cais do Sodre. There you can hop on the metro at Cais do Sodre metro station, which is on the green line. Take a metro direction Tehlheiras and get off at Rossio (just 2 stops), in Lisbon city center. From there you can take take a train from Rossio train station if you want to go to Sintra.