I agree with everyone else. Definitely find a trainer BUT!!!! Find a good one. Spend time with them, watch them work with other horses, see how they treat their animals. My family spent almost $1000 for my appendix to have 60 worth of riding and when he came home he chucked my dad a month later, then was unridable for another 3yrs until we spent $100 for a group of crazy boys to ride him for a week and now he's a great trail horse, but thats because they actually rode him. We believe my trainer was in 'kahoots' with a roper from TN who wanted to buy Joker but I turned him down. Some trainers will SAY they have broken your horse to ride but they actually haven't so that when the horse freaks out on you, you look to them to sell it. They get a "finder's fee" and you get swindled so BE CAREFUL!
Also, in my experience, a 4yr old horse is never a good idea for a 12yr old, especially one thats never ridden before. If you can, find an older, deadbroke trail horse and when you go to look at it ask the seller:
Why is he for sale?
Has he ever been sick?
Does he have any vices?
Has he ever been injured?
How is he with other horses?
Does he kick\bite\rear?
Will you ride him?
and make darn sure that they walk, trot and lope the horse in question and watch for any signs of aggression or distress i.e pinned ears, swishing tail. If you can, have a very experienced horse person that you trust go with you from now on to view any more horses OR to pick out a trainer.
Four things greater than all things are:
Women and horses
And power and war. - Rudyard Kipling