From barcelona, go to the sants station and get your train ticket. Sants is easily accessible from the metro lines. P.S. get a t10 for the metro rides in barcelona. Buy the park ticket AND the train ticket together, its much cheaper BUT note, they do not accept credit cards, only cash. T10 pass 7e good for ten trips on the metro the ticket to port aventura 10e but its like 35e for teh trainplus park ticket. great value. The train station at port aventura is not really a station just a stop with some benches so make sure you get a return ticket. Sants station is VERY confusing especially if you dont speak catalan so please do not do like us and get on the wrong train.
If your in barcelona you must visit tibidabo. I am glad we did. The metro system well take you part of the way there (Using your t10 pass) then you pay 3e for the funicular to the park. the funicular doesnt take dredit card either but the park does adn will reimburse your money from teh funicular. The walk from teh metro to teh funicular is long and uphill if the trolly isnt working. Trust me the park is worth a visit, think of it like willy wonka and the chocolate factory!