By plane: the closest airports to Sitges are the airports of Barcelona.
The main international airport is Barcelona El-Prat Airport, served by most large international carriers. Guidelines regarding metro, train and bus connections to the city of Barcelona are available in detail on the website of Barcelona El-Prat Airport. You may use the information therein in combination with the train timetables from the Spanish Railways official website.
Low-cost international carriers (e.g. Ryanair) and other seasonal international carriers serve two smaller airports close to Barcelona: Girona-Costa Brava Airport and Reus Airport, each located about 100Km away from Barcelona, to the east and west, respectively. From Girona and from Reus airports you can reach Sitges or the city of Barcelona by special bus services (please consult the webpages of Girona-Costa Brava Airport and Reus Airport).
By train: detailed information on how to reach Sitges by train, from the city of Barcelona, or from the airports of Barcelona, can be obtained from the Spanish Railways official website. Therein you can book train tickets as well. Additional information regarding the trips by train from the surroundings of Barcelona to Sitges is available also on the touristic webpage of Sitges.
Timetables for the green line R2 from El Prat Airport to South (i.e. to Sitges) and back, can be found also on the airport's webpages .
By car: depending on which airport you travel from, you can find very helpful guides to reach Sitges on the touristic webpage of Sitges, indicating as well where highway toll-charges are applicable.