While this post might be obvious to many, mostly to those living in cities, and especially here in Europe, it might be not so obvious to people living in different environments elsewhere. And the proof is the number of messages (and PMs) I have seen in different forums regarding this issue.
This applies to Barcelona... and, mostly, in any other city or town in this part of the world :)
HOW TO READ AN ADDRESS?
Type of street - Street - Number - Level/storey - Flat number.... so, for example, an address like "Rambla de Catalunya, 145, 3r-2a" would be a flat located on a "rambla" (a type of street which is semi-pedestrianised) called "Catalunya", at building number 145 and the flat would be door number 2 on the third floor/level/storey of that building.
Street plaques are located ON THE FAÇADES OF THE BUILDINGS in the corners of the streets, like this or like this other. They're on black letters on white marble plaques. Sometimes, depending on the available space, the plaque might not be "right at the corner", but a few feet from it, but always on the building.
Even numbers are on the plots on one side of the street and odd numbers in the other.
Be respectful with the names and always use the full name (no abbreviations, no shortening, etc.) since, for example, it's not the same Rambla Catalunya, Passatge Catalunya or Plaça Catalunya and sometimes they can be miles apart. Furthermore, respect the whole name: "Carrer Bonavista" and "Carrer de Bonavista" are two different streets.
I take the opportunity to remind the same applies for names of places, please no shortening, otherwise names cease to make any sense for us locals (ie. "Sagrada" is not a place, "Sagrada Família" is... 'sagrada' just means sacred, and there are several religious buildings sharing that generic word.)
A NOTE ON LANGUAGE
(applicable in Barcelona, or anywhere else in Catalonia, as well as in Basque Country)
Some sites, often run by certain Spaniards with political agendas or by expats that didn't bother to realise Catalan is "the" local language in Barcelona (and Catalonia) and not Spanish, they do translate the street types and names into Spanish in their websites and brochures. Be aware that the street plaques in Catalonia are written only in Catalan (obviously!).
One can still find the odd old plaque from the Franco dictatorship era (1939-1975) when streets were forced to be "hispanized", and some were even renamed to extol fascists of the rebellion that led to the dictatorship, but these 'remains' are more likely to be found in very small rural towns with no budget to change the plaques than in major cities.
While both, Catalan and Spanish, are Romance languages hence they share similarities -as with other Romance languages like French, Italian and Portuguese- this is not always so, and some words are completely different. For example, in one of the central neighbourhoods in Barcelona, there are a bunch of streets named after medieval guilds, such as tanners, shoemakers or blacksmiths among many other, but their Catalan names (blanquers, sabaters, ferrers) have little in common with the Spanish translation (curtidores, zapateros, herreros).
I am saying so because if you were given a name you can't find on a map (or in-situ when you're visiting), you should check on your Google Translate (Spanish > Catalan) whether you might have been given the name in the wrong language. As said, Catalan is the local language and is what you'll find in the street plaques and other street signals. Fortunately, Google Maps has already updated most (albeit not all!) of its database and now the names are correctly indicated, in Catalan language, reflecting what you'll find when on site.
Enjoy