Nice that you are planning for a horse at 13! I spent much of my teen years designing barns, lol. Had to keep changing them as my needs/desires changed.
Just some general guidelines to help you out that I have learned over 40+ years of horse ownership:
1) figure on two acres per horse depending on how much grass there is and the quality of the grass.
2) Never have only two horses as they can become too attached and make it nearly impossible to ride/enjoy them unless you always have a second person to ride the other horse.
One or three horses minimum. Contrary to popular opinion, a horse can be quite happy living solo, depending on their temperament. My current mare enjoys her free time when the two geldings go off to ride.
3) Put your shelter on a slight raised area to make it easier to keep the floor dry. Their feet need a dry place too.
4) design your shelter with a way to close the horse(s) up if you need to. Might have an injury, want to keep them clean for a show, or the most common...to be able to control each horse's food intake. Rare to find horses that eat at the same pace and keep their noses in their own bucket!
5) Best to have blockage from storms, strong winds & rain instead of just a three sided shelter.
6) make sure less dominant horses have a way out. Worst mistake is to have a stall in a pasture with only one doorway.
7) Horses will come to the shelter more if it is in a good spot. It is very hot most of the year here in the south, so I located mine in a shady spot with trees nearby. It is nice an cool in there during the day, and my horses spend most afternoons inside.
8) A barn/shelter can be built in stages. The most inexpensive option I have discovered is a metal carport with 8-10 foot legs. Buy the largest one you can afford, then as time goes on you can build in stalls/tack room/feed room/equipment storage/wash racks...whatever you discover works for you.
9) Always plan for more space than you think you will need! Very easy to acquire horse stuff
Have fun dreaming and designing your perfect place!!