The biggest challenge when changing where one lives is finding people to do things with. There will always be things to do, even if they are new.
Many students go on their study abroad with a friend in the same program. This makes their experience much different from those who go not knowing anyone else in the program. Often if a student comes with a friend, they will find less reason to make new friends, and so do not get as many opportunities to meet individuals from the country they are studying in.
Among all US students, especially those who choose a study abroad in Barcelona, there are a number of extreme party-goers. They go abroad to drink, dance, and drink some more, in large groups of Americans. They are usually very sociable individuals, who make friends with everyone they meet at the clubs very quickly.
I fall neither into the category of coming with friends nor in the sociable party-loving. Like a select few students, I am shy, timid, and do not like staying out all night when I have classes the next day. If you are like me, do not despair! You are far more likely to make friends among locals if you are not surrounded by a pack of American students. Being in classes with locals means an opportunity to make new friends and practice the local language! And going out on your own to local establishments (bars, cafes, clubs, etc) can be another great opportunity to meet new people.
Of course, you will find other like-minded American students to spend time with in your language classes or on program excursions. But do not feel limited by your circumstances! Being shy, or less interested in nightly parties will not be a problem for making friends if you make an effort to talk to people.
The fastest way to make a new friend is to find out something the person likes and get them to talk about it. Hobbies, sports, family, whatever. Be willing to share a little about yourself, but even more ready to listen to what they have to say about themselves. It will go a long way.