I really recommend a trainer. The only tips that we can give you are to show up on time, make sure you have all the things you need, bring a helper, make sure your horse is very clean and tidy and make sure that you are clean and tidy.
We don't know what disciplines/classes you have available to you, your riding education or your horse's training background. Shows have a lot of little nuances to them that we just can't give you because we're not there and don't know the shows. A trainer can help you through these things - from the little things of the best places to park at a particular location, to walking you through the paperwork, to holding your hand through the whole process and helping you train for it all beforehand. A trainer is going to be able to help you decide what classes you should enter and how to train for them. A trainer will know the judges and what they look for. A trainer will know what all the classes mean and what tends to score well.
If you're just showing "for fun" and not to win, then most of that doesn't matter. Figure out what classes you want to enter, show up on time, neat and tidy and go for it. If you want to win ribbons, then you have to put in work.