Catalan is an Ibero-Roman language. In scientific research you will most likely learn that Spanish – or Castillano as it is usually being called here – and Catalan are not close enough for speakers of one language to understand the other, without any studies. Personally, I believe that if you speak Spanish and just accept to be exposed to Catalan, you will automatically develop an extensive understanding of the high degree of systematic difference between Catalan and Spanish grammar as well as vocabulary. Once you start to see the similarities, you will have it easier to identify and learn those words and structures which bear no resemblance at all.
Maybe it is because I live here in Catalonia, where both languages are being used, and not in a mono-lingual part of Spain, but I do think that a big part of the explanation to the lack of natural understanding between Spanish and Catalan is rather political than linguistic. Many Spanish speakers are happy to tell you how well they understand Portuguese and Italian without having studied the languages, but at the same time claim not to understand Catalan. This, to me, reveals that they see Catalan rather as an unnecessary statement of regionalism than merely as a language.
Unlike some tourists tend to believe, Catalan is the thriving, dominating language in North-Eastern Spain. If you listen to how local people speak, you might hear quite a lot of Spanish in the tourism and business areas of Barcelona, but not as soon as you come out in the rest of Catalonia. Vilanova is a good example. In our local supermarkets my family is usually the exception when speaking Spanish to the staff – the people in front of us as well as those behind us will be speaking Catalan. In spite of the many national Spanish speaking radio channels, broadcasting in Catalan totally dominates ether media so it take more energy to turn away from than tune in to Catalan radio or TV.
With this in mind, it is surprising how many people come to Barcelona to study Spanish. Among foreigners in Catalonia there is an evil joke that Spanish is such a popular language that the courses are expensive and always full while courses in Catalan have to be given for free. Well, this is not so strange since outside North-Eastern Spain the value of learning Catalan is limited. Here, however, knowledge of Catalan is needed if you want to integrate into society and as foreigners who have decided to live here, let us be grateful that the Catalan authorities are so generous. There are several dictionaries available for free on the Internet and many centres with special sections for self-studies of the language. In Vilanova, for example, there are two libraries of this kind.
However, if your spare time is limited and you try to learn Catalan through everyday interaction with local people, this is not easy at all. That is at least my experience. People here are social and helpful, so when you try to address them in Catalan, as soon as you make your first mistake in a sentence or need to replace a word with the Spanish alternative, you can be sure that the person you talk to will change for Spanish. Although the intention is good, this efficiently hampers opportunities to practise Catalan in daily life.
It is a bit of a contradiction that I found it easy to learn Spanish here in Catalonia through that method. Since few people here speak English, Spanish is the natural choice when speaking to foreigners and local people do not mind having to use their second language when communicating with you. Luckily enough this issue has been identified by the generalitat, so there are in fact a program matching foreigners interested in speaking Catalan with local volunteers who are strict in not answering in Spanish.
Still, what I would ask for is for Catalans to be more patient with us foreigners when we try to speak the language. Almost all of us master Spanish better than we speak your language, so when we try to do so, it can be assumed that it is with an intention to learn rather than by necessity. Since you are usually proud of your language, help building the knowledge of it with us new speakers.
Finally, in order not to be misunderstood, let me point out that I only speak for people who want to speak Catalan. Whenever the answer is given in Catalan when a foreigner or fellow Spanish citizen addresses you in Spanish, do understand that you will not come across as someone who is promoting his language, but rather as a rude person trying to avoid communication. That label certainly does not improve the outside world’s picture of Catalonia.