Finding good instructors can be very tricky, especially if you are young and inexperienced. That being said, I agree with Fulford. Every instructor has something to offer, be it teaching you what NOT to do, and what types of instructors to stay away from, or teaching you how to ride properly.
I was blessed, when I was younger, to have had an array of instructors, both good and bad. I jumped around barns a lot and rode rather sporadically, as my parents couldn't afford much. The first barn I ever rode at was an up and coming hunter barn that wasn't much when I started there 15 years ago. They gave me a very solid hunter seat and basic riding/handling skills. Now, 15 years later, they are a huge operation and doing very well. After them, I had the misfortune of falling into a few bad places that took advantage of my experience by sticking me on green horses and mostly ignoring me during group lessons. These were mostly summer camps though. Soon after, I was blessed to find an incredible barn in Virgina where I learned all of my advanced horse handling skills and was turned to eventing. Having been riding for 15 years, and having jumped around quite a few places, I stand by the fact that I learned more at that barn than I have anywhere else. Since then, I have found that not every instructor works for every person, and you shouldn't write them off as a bad instructor simply because they don't work for you. My most recent trainer taught me, I dare say, nothing, but I was blinded by friendship, so I rode with her. She was not a bad instructor, per say, she's great with kids and beginners, but is not as great at helping the advanced riders along. That being said, she was more than happy to take money from the people who were learning nothing from her, so be careful!