There is no registration required in Spain, although it is wise to apply for your Certificado de Empadronamiento. This is a kind of local registration, and everyone in Spain (citizen or resident) should be registered in their local area.
Because the local government is given money to provide services to everyone in their community, the more people in their district who register, the better off they are. Because of this, everyone is encouraged to register even if your paperwork is less than adequate. All information is confidential and even if you are illegally in Spain, you will still be entitled to free emergency healthcare and a local healthcare card if you have the Certificado.
It is a simple process which requires you to go to your local town hall (Ayuntamiento), show proof of your address, your ID and fill in an application form. You will usually receive confirmation on the spot, otherwise it will be mailed to you within a week. Having the Certificado also makes life a lot easier when you want to apply for residency and for almost all paperwork purposes in Spain.
The main Ayuntamiento in Barcelona where you can apply for your Certificado is on the Av Parallel, 202-204, Sant Antoni, Eixample, Phone: 932562700. Only use this address if you live in the middle of Barcelona – there are Ayuntamientos dotted all over Spain and you will find that the one nearest you is likely to be much less busy. (You can refer to the following website: https://w30.bcn.cat/APPS/portaltramits/es/inici/default.html but keep in mind that it’s only in Catalan and Castellano, so seek help if your language skills are not yet proficient).
Spaniards speak easily with strangers (its common to bid good morning to everyone when you get on a bus) so don´t be shy. Just point to your street map and say the word ´Ayuntamiento´ with the inflection of a question.