(2-part post):
It's probably not impossible but it's going to be very difficult.
Travel agents and package holiday operators won't normally accept bookings from people who're under 18, because you need to be that age before you can enter into a legal contract (which is what a holiday booking is). Even married couples, who're 16 or 17 years of age, can experience difficulties in booking holidays.
It's theoretically possible to book package holidays if you can persuade a parent (or any other adult) to act as a guarantor but, in practice, most companies simply have a blanket rule which will prevent you from travelling with them
Your best chance of arranging a holiday is to put your own package together. Most budget airlines would permit you to travel. On Ryanair, for example, the 16-year-olds could fly without restrictions but the 15-year-olds would need to have a parent travel to the airport with them, and remain at the airport until the flight has departed:
http://www.ryanair.com/site/EN/faqs.php?sect=c hd&quest=uam
(NB: Don't change your destination from Spain to Portugal because, as that link shows, you'd almost certainly be prevented from entering Portugal).