Tibidabo is located on the top of the mountain behind Barcelona. There is a large chruch on the top overlooking all of Barcelona and it's surrounding cities.
The view is spectacular but be warmed there are a lot of stairs!!
It cots 4€ for the funicular (a type of railcar that goes to the top of the mountain), 2€ to pay for the elevator if you want to go all the way to the top of the church, and cost of ride in the metro.
The easisets way to get to the top I found, is to take the L7 (brown line) until the last stop called "Avinguda Tibidabo" and then there's a type of bus stop for the Tram or if the tram isn't running a bus will come. The stop is across the street from the exit of the metro but ask around and

they will point you in the right direction.
If the funicular isn't running and you're not sure about the bus, DO NOT try and walk. Trust me I tried...
If you have the chance to be in Barcelona after it rains that would be the perfect time to visit the church because the view is cristal clear.
Thanks Toby Hammond for the tip!!