Search up barns on the internet and you're sure to find some. Check the prices first, and choose some that are in your price range. Then, schedule a lesson with the first one. Go to that place and see how you like it.
Meet the horses and the instructors. The instructors should not be young or your age, because that means that they're be inexperianced. The horses should be well trained and fit. It's not a good idea to ride at a barn full of fat and untrained horses. Pay careful attention to how the horses are treated. If all of the horses are overweight, it means that they are either being fed too much (or the wrong type of feed), or they aren't being excercised enough. Watch to see if the horses are beaten/abused, because that's never a good sign.
Taking a lesson there will show you how good the instruction is. From your previous knowlage, you should know basic riding positions (I'm assuming). See if the instructor(s) catch your mistakes. You can even purposely make a position error and see if your instructor notices.
If you don't like that barn, move on to the next one and try it out. Keep doing that until you find one you like. Good luck.