I think from experience, book into a hotel for 1 or 2 weeks, then look once you are here, as there are alot of scuzzy places for rent.
It is best to actually see what is being described as the most wonderful apartment, area will be important too, try & get the best your budget will allow if you live in a bug hutch you will be miserable & move, costing more in the end. Also make sure you get a proper rental contract, it will be in Spanish, they should run you through it if they are good.
Work out what your budget is then look, try & get bills included too.
Good luck, hope this helps