Horse shows aren't for everyone. There is a lot of prep time, pressure, goals, and lets not forget they aren't free.
But there are many benefits to showing. Even small local shows give riders the opportunity to work towards tangible goals, and to show off their skills and be proud of their achievements. There is also the added benefit of another professional horseperson watching your ride and objectively judging your skills. At local schooling shows, most judges are willing to talk to you after your ride to point out areas for improvement. You can then work with your trainer to improve, and sometimes hearing it come from someone else in the same or a different way, will help to solidify what your trainer has been teaching you.
Preparing for a show is an extensive process. First of all, you have to make sure that your horse can handle the added commotion. This means you can work with your horse on being confident in crowds, with unfamiliar objects, and unfamiliar people. Sometimes just hauling to a trail ride with a group of horses can show you where your horse excels in groups, and where you may need to focus more effort and positive reinforcement.
Once you are confident your horse can handle unusual circumstances, crowds, added noise, and hauling to a new place.... then you can focus on you. Talk to your trainer about what type of show you should try, what classes you should enter, and if he/she will come along to help it go smoothly. You shouldn't think of a show as competing at the highest level possible. A show should be easy, fun, and a manageable challenge. For example, for a novice rider, schooling jumps at home of around 2'6", your first show or first few shows should be at a level that is easy, and lower than what you practice at home. So you may consider showing at 2' or 2'3". Its very likely that you and your horse will be "amped up" the day of the show. You don't want to add the pressure of jumping at your maximum level.
Judging shows is not a science. Each judge has the right to choose how important riding errors are in placing a class. You may find a judge who pins a rider last because she missed a lead, or was late changing her diagonal. Another judge may allow that rider to pin higher, over someone who has a floating leg. This is another reason talking to the judge after your class can be so beneficial. There is always something to learn at a show.
Beyond all the training, horse spent in the saddle, a show also almost forces you to look at your horse, your tack, and your organization skills under a microscope. Your trainer can help organize all the items you need.... but a short list includes: water bucket, hay, haybag, bridle, saddle, show pad, girth, spare bridle, brushes, show sheen, fly spray, negative coggins test, boot polish, tall boots, show breeches, show coat, hairnets, helmet, treats, leg wraps or shipping boots, hoof polish.... the list can go on and on and on, depending what type of show you are going to. All of this has to be prepared and loaded into the trailer (Don't forget the horse!!), often with a departure time of 5-6 AM!!! Bring a mug of coffee, and don't forget to smile!!!