Folks, I have to disagree to some extend. If it's just trail riding mostly at the walk, and you are naturally gentle and have enough experience to hold balance in western saddle, lessons are not THAT important. They are still very BENEFICIAL - no arguing on that, but they are not A MUST.
I know number of people who got nice quiet trained horses, they can WTC without falling off and keep reins loose 100% time. All they do is 3-5 times/week saddle up the horse and go on 1-2 hour trail ride (mostly walk). Why would you need lessons for that?