It's the safest way to travel. Far more people die in car crashes, but that's not news because it happens so often. Plane crashes only make news because they're rare and there's so many more people on a plane than in a car.
Also bear in mind that the plane gets serviced far more often than your car (if you have one). Every night, in fact. If there's anything dodgy with it, it ain't gonna be flying again until that gets fixed! The regulations about this are just so incredibly strict. And the pilots (there are always two so you're not reliant on just one) are not going anywhere unless they feel safe. They want to get there in one piece too! So do the flight attendants.
And if the worst comes to the worst and you want to go home and can't face another flight, this is Spain. It's not as if you're crossing an ocean to get there. You can always get home by train (including the Channel Tunnel), or train and ferry. It'll cost money and you'll probably lose what you spent on the flight home, but it's a plan.
Do NOT resort to alcohol to feel better about it. Drunks are boring. I know - I have a mate who tends to do this on planes and it's bloody annoying.
It'll be great. You're with mates who you presumably like being with anyway, they're not worried, so just enjoy it! I was 8 when I first flew (family holiday in Majorca) and that little boy was so excited about being on a plane. Something new and different... at that age, that was all I could think about, and I loved it. 44 years later, I'm still the same. I want my window seat and still get fascinated by it all. Can you think like that about a new experience? I hope so!