For more precise and detailed information, I think you may search at www.baidu.com for 儿化音,or you may search and download (for free ;) books such as 普通话教程, 普通话培训. These books are intended for Chinese people who're learning Putonghua, but will help foreigners with R sound as well, I suppose.
It's true that the major Chinese population ( including a great portion who speak northern dialects) do not have the R sound in their own languages and therefore they do not pronounce the R sound even when speaking Putonghua. The reasons supposedly are 1) habits 2) they donot know where should be and where should not be an R sound in Putonghua 3) Some people have difficulties in making this sound, especially for the more southern people, because there is no such a or similar sound in their dialects.
But the R sound does not come disponsible in Putonghua. That's a must for learning standard Putonghua. (For more information, please consult the above reference). < But, as you know, lots of Chinese people do not prononce this sound very well when they speak Putonghua, so it does not matter so much actually, on the other hand ;)>
Chinese linguists has long been trying to make a R-sound word list as national standard, but not yet achieve it. The problem is, although R-sound is necessary in many words, it's otherwise optional for some words: it's depending on the individual habit when its disappearence does not change the meaning of the word.
I think you would not have difficulties in making the R sound, for you come from Spain, and there are similar sounds in your languages. In fact the Putonghua (Vowl)-R is pretty the same as that in the American English (GA). eg paRt, letteR, giRl. What's different is that when we pronounce Putonghua R we do not curl up the tongue edges.