Traveling to Barcelona need some help please!?

yes, you are right that they will not let you stay in the hotel as you are underage! You can make some of your friends or relatives who is at least 18 to make the booking for you but the thing is that the person who makes the booking has to accompany you because he needs to prove that he/she makes the booking! This is the best you can do! You can also contact the hotel where you want to stay and ask them for advice!