The Joan Miró Foundation is situated on the hill of Montjuïc and is a modern building with spectacular views on the city. It is closed on Mondays and the entrance fee costs 8€ (6€ with student discount).
I recommend the audioguide, very interactive, with comments either by painting or room, and it will provide you with a lot of very useful information to get to know the work and personality of this character better.
Miró was born at the end of the 19th century and started to paint very early. In the foundation there are very old paintings of when he was still a teenager and went on holidays to France, paintings landscapes. They also display paintings and drawings of his trips to Paris, where he met Picasso, various painters, artists and French poets. His masterpieces are also displayed in the foundation, which turned 30 years old in 2005, and many of his works are spread in the streets of Barcelona, like the mosaic of Las Ramblas or the sculpture behind Plaza de España in the direction of Sants.