Recommendations for minors traveling out of the country?

Okay so at the end of July my friend and I, both 16 year old females from California, are traveling to Madrid, Spain. It will be just us and we will be staying with our friend who lives there. I have been out of the country 15+ times, whereas she has never. So to be in a safe, and a secure traveler, do you have any recommendations being that we are minors? Thank you in advance.