Tibidabo is the highest mountain in Barcelona.
Climb here and you will see the Temple and vista point, have fun in Luna Park, and enjoy an amazing panorama of the whole of Barcelona and the whole area from a height of 512 meters.
The easiest way is a taxi. The ride from the city center will cost you 15-25 euros one way.
Train+funicular+bus is not the easiest but a much more romantic, colorful and cheap way to get to the top of the mountain. Take the S1-S2 train at Placa de Catalunya. It takes you to the funicular station called Peu del Funicular. Take the funicular to the bus stop, and there take bus number 111, which runs every 10 minutes and stops right in front of the temple on Mount Tibidabo.
Do not forget about comfortable shoes and plenty of water.