It's recommended that you start looking at NCAA riding teams during your freshman or sophomore years of high school. There are only 23 colleges to choose from, so your odds aren't all that great (source). So get cracking. Do your research and—if you can—check out an NCAA Equestrian competition to see what they're looking for.

Also, get your grades up to at least a 2.3. The NCAA doesn't accept anything less (source). And don't forget to tell your trainer that you're trying to get a scholarship so they can write you an awesome letter of recommendation.

Getting an NCAA scholarship is a lot like trying to get a job. You need to send in two resumes: one with your non-riding accomplishments (like how you're a straight-A student and you volunteer at the soup kitchen on Sundays), and another one with all your show records, your trainers and coaches, and any organizations you belong to. Keep careful track of your expenses and any prize money you've won so the school knows what to expect (source).

You'll also want to put together some high-quality footage of your best moves on at least three different horses. Make sure you're wearing show-worthy gear and that your horse has been thoroughly currycombed when you shoot; you want to make a good impression on the coaches (source).

What you do in the video will depend on whether you do hunter or Western. Hunter riders will want to show a lot of flat work (dressage, sitting trots, etc.) and some jumping, with special focus on how you ride the horse on the bit. Western riders should focus on walking, jogging, extended jogging, and loping, as well as practice patterns and reining maneuvers (source).

Finally, you need to register online with the NCAA as an amateur athlete to qualify for one of their scholarships (source). You should also go the NCAA Equestrian colleges' websites and fill out their Prospective Equestrian Questionnaires.