9 times out of 10, green on green equals black and blue. I'm not saying that it hasn't been done, but for a 15 year old first time owner to take on a yearling and train it alone is begging for trouble. Please understand that I'm not trying to be mean or anything, just that an unbroken yearling training project is not an appropriate first time owner's horse, no matter how old the person is.
Firstly, how much experience do you have with horses in general? Have you taken riding lessons? For how long? Do you have a friend or family member who is into horses, who you ride/work with? Have you ever leased a horse? Have you ever worked with a youngster before? An older but still green horse? Seen/helped with the colt starting process in person? These are all experiences worth having before diving into the responsibility of horse ownership, even if you are looking at an older horse, much less a youngster.
For anyone's first horse, I would recommend a few basic guidelines.
Aim for a minimum of 5 years of age. Many five year olds have a solid start and handle on them, but the downside is that some still have a little of that youngster left in them that can make them a challenge for a novice owner to deal with - this of course depends on the individual animal. Some 5 year olds are real steady-Eddies, and some 25 year olds are nigh uncontrollable. The ideal range would be somewhere between 10 and 15 years. In that range, the horse can be generally expected to have a good handle, plenty of miles under it, and has reached an age where its temperament is pretty well settled.
Look for a horse that is already trained for what you want to do. Nothing drains the fun from horse ownership than yearning to get into the showpen or hit the trail when you've got to deal with basic training for another couple of years, or deal with problems. A good first horse is solid and controllable on the ground and under saddle at all three gaits, halt, rein back, and basic lateral movements (turn on forehand/haunches, leg yield, sidepass). Add to that good ground/stable manners. The horse should lead respectfully, load into the trailer without a fuss, and stand like a statue for the vet/farrier/equine dentist.
I very highly recommend picking up a copy of this book and reading it thoroughly: http://www.amazon.com/gp/product/1585747149/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B002AS5BYK&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=097N9KWYE0RSZ8327331
It is a much more comprehensive guide for the first time horse owner than can be typed on a forum. Do your homework, find people who can help you in person, whether friends, family, or a good trainer/instructor. Look for a horse who can teach you something, not one who needs to be taught everything. At fifteen, you have an unthinkable amount of time left to delve into colt starting when you have more years of experience under your belt, and you have more tools in your toolbox to paint a solid, soft, light responsive, and willing partner onto that blank slate.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown