There are plenty of options, I believe. You can catch a train that goes to Tarragona and change for a faster train to Sitges there, or catch a train that goes via Tarrgona to Sitges as well. Most trains from Salou will pass through Tarragona, I think, and so you will find a train quite esily which stops in Sitges. It isn't that far up the coast from Tarragona to Sitges, so taking the cheaper, but slightly slower train is the best option IMHO . I never use the trains, so I can't be more precise than that,..sorry!
Nick, the train from Salou hasn't stopped at Sitges for some years . You would have to get off at San VicenÇ de Calders or Vilanova de la Geltru and catch another train from there. Not sure about the bus service but don't think there is a direct one. Let me know if I can be of further help