I have a 10yo horse loving DD who got bit by the horse "bug" around age 6.
As far as bedroom decor look at "Company Kids" for beautiful horse themed bedding, curtains, rods, shelves, lamps etc... My DD LOVES their stuff and so do I, so cute!!!! They also have horse themed luggage, backpacks, and bags.
My DD has a subscription to "Young Rider" that has posters and great horse pictures that she hangs all over her room.
She also collects and plays with Breyer model horses. She has several lines of them - some are "show" and others are playable.
Lego also has a horse trailer, truck, horse and rider set that I got DD and now she wants the whole barn etc....
DD loves to read horse books, "Black Beauty", "Misty of Chincoteague", "Stormy" etc. etc... When she was younger she read the "Keeker" series of a young girls who rides.
Oh and great game is "Ponyoply"!! She also has the horse stamp set someone else mentioned and she also has horse stencils, folders and notebooks. We have a Dover Saddlery right near us so I always get her a horse themed t-shirt for her b-day or Christmas.
Lessons are also great for a horse riding girl. DD has taken them off and on over the last several years and she would love to right now too however being on dance team takes quite a time committment plus violin lessons so we do horse lessons in the summer time. I would advise you NOT to start lessons in Dec. Although many stables will do winter lessons I do not think its a good experience to start in the winter, its just too darned cold. Sometimes the horse mixes the shivering legs with the signal that he should be moving and the riders have a harder time controlling the horses. Plus those inside rings I think are colder than being outside where the sun at least gives off a little warmth. DD did one set of lessons in Dec. and has since sworn off winter riding despite special lined riding pants, boots and gloves it was just too cold to enjoy for her. But she loves spring, fall and summer riding.