Hi mermaid. You may find that although you are not 18 yrs, as long as you have written consent from parent/guardian to travel, this is accepted by travel agents/travel companies as this is the legal age for the holiday contract.
On the basis you are a responsible 'young person' you can get a booking with a parent signing the documents - insurance may not be avilable to you either by the way, so look out for that too.
this is a parental consent form for a UK holiday in Cornwall and could be similar to one you may be asked to have signed - note there is a bond to be paid too -
Soverign Holidays and several others will take the booking on the basis that you have the consent.
Youth hostels are not about dormitories and old socks nowadays - have a look at Spain info
here and maybe this would suit as puzzled says.