For an up to date timetable for the bus or the train, your best bet is to get one from the Tourist Office. Times change frequently because of holidays and special Saints days.
Make sure you know the last departure time from where you are going, back to Salou, because they can be suprisingly early. However, armed with a timetable, the local buses are a cheap and (sometimes) fun way to travel.
Departures from Salou to Cambrils are very frequent, about every 20 minutes. Look out for the red bus stop signs on the side of the road, especially along the seafront, on the same side as the shops.
You can also catch a bus to cities of Reus or Tarragona, both journeys take around 20 minutes. A central bus stop to both places is just off Placa Europa near the Tourist Office.
Probably the easiest way to Barcelona, is by train, the journey takes 90 minutes, catch the train in the old town of Salou.
The first train leaves from Salou at 7.07am, with quite irregular departures throughout the day until 8.37pm
The last train from Barcelona to Salou is at 9.00pm!!!
Remember to leave yourself plenty of time, as Barcelona is huge. Also, Pickpockets love the hustle and bustle of trainstations. REMEMBER if you are unsure of times, platforms etc, ASK, they see confused visitors every day, most of the station staff speak English. Your not alone.