I don't know if this is the right place to post this or not, but I'll give it a try.

I'll be flying from minneapolis/st. Paul to Jerez de la frontera in Spain. I have layovers in Dallas and madrid. Here are my questions I need answered. (Btw I'm 15 years old but I will not be traveling as an unaccompanied minor. This is my first time flying solo)

When I'm getting my luggage checked in at MSP do I need to show my passport? Will my boarding pass/itinerary show that I'll be traveling internationally after my stop in Dallas? Same when I'm at security and boarding the plane, do I need to show my passport at MSP?

When I land in Dallas what do I do? I'll be taking the same airline for the first two flights, so I won't need to recheck my bags right? Do I just stay in the terminal and find my gate for the next flight? Do I need to go through security all over again? Where do I show my passport?

When I land in Madrid will they just let me off the plane and will I be able to stay in the terminal and find my next gate? I don't have to go through customs, right? And also since I'll be taking Iberia for my flight to Jerez de la frontera do I need to recheck my baggage or will hey transfer it for me?

When I land in Jerez de la frontera what do I do at customs?

Sorry for a the questions but this is my first time traveling solo, and my second time flying internationally.

When I'm getting my luggage checked in at MSP do I need to show my passport? YES

Will my boarding pass/itinerary show that I'll be traveling internationally after my stop in Dallas? YES

Same when I'm at security and boarding the plane, do I need to show my passport at MSP? YES, FOR SECURITY (YOU NEED A GOVT-ISSUED PHOTO ID.) NO, AT THE GATE.

When I land in Dallas what do I do? GO TO YOUR GATE. THE U.S. HAS NO EXIT FORMALITIES.

I'll be taking the same airline for the first two flights, so I won't need to recheck my bags right? RIGHT. THEY SHOULD BE CHECKED ALL THE WAY THROUGH.

Do I just stay in the terminal and find my gate for the next flight? YOU MAY HAVE TO CHANGE TERMINALS. INTERNATIONAL FLIGHTS LEAVE FROM DFW TERMINAL D. DFW HAS AN ELEVATED TRAIN SYSTEM CONNECTING THE TERMINALS, ALL INSIDE SECURITY.

Do I need to go through security all over again? NO

Where do I show my passport? YOU MAY BE ASKED TO SHOW YOUR PASSPORT AS YOU BOARD YOUR FLIGHT TO MADRID.

When I land in Madrid will they just let me off the plane and will I be able to stay in the terminal and find my next gate? YES, ALTHOUGH YOU WILL NEED TO GO THROUGH SECURITY AGAIN.

I don't have to go through customs, right? YOU WILL GO THROUGH IMMIGRATION (PASSPORT CONTROL), SINCE IT'S YOUR FIRST POINT OF ENTRY IN THE EUROPEAN UNION.

After this where I get murky since it's a flight within Spain. Customs at Madrid or Jerez? Not sure.

I'm distinguishing between immigration (passport control) and customs (baggage inspection) The first will definitely be in Madrid. The second ... someone else will need to answer.

As JCR wrote, passport control will happen in Madrid before you get to your gate to Jerez.

"Customs" (baggage) will be in Jerez.
It is only random spot controls. You simply walk out from baggage carousels through the color-coded exits: green/blue "nothing to declare"/ arrivals from the EU" or red "goods to declare".

I'm surprised your parent(s) or guardian(s) are not helping you answer these questions!

My daughter flew thru Madrid alone when she was your age. One thing she still mentions is that people in Spain don't get to departure gates and line up well before the flight time. She thought she was at the wrong gate since it was empty 45 minutes prior to flight.