Green on green makes black and blue
Translation: An untrained horse with an untrained rider is asking for world of hurt.
Besides the disappointment of not accomplishing your dream, you are placing yourself in great danger. First graders can’t teach first graders. If you are a novice, ask a knowledgeable horseman to help you select a sound, well-trained horse to start with. Then, take riding lessons to learn the right buttons to push.
Or, take lessons on a well-trained, reliable trail horse. The big advantage to this approach is that you can concentrate on improving yourself. You won’t have so much to figure out, training both you AND your horse.