Welcome. If you're looking for advise to start collecting items like you've found, you're on the right place and you'll get a lot of information, just posting images of the items you've.
But if (as I guess) your main (or only) aim to be here is to dipose of the treasure that you've found, I would rather suggest to contact stamp dealers in your own country (tipically in Madrid or Sevilla), show them your newly found treasure and see what they offer for them. Should they be interested.
Maybe, you could tell us from what country your first day covers are.
It makes sense to contact a Spanish dealer if they are Spanish covers. But can we infer from your statement that you are from Málaga that your covers are Spanish covers? If they are not, a Spanish dealer may have little interest if Spanish collectors do not collect the country or countries your covers are from.