First, make sure you have a target audience (english pleasure, H/J, dressage, grass roots, more professional, etc..). Then, check to see with the USDF to see what kind of licensing you need to do, if any.
I run dressage shows and make money at it. I run only sanctioned/licensed shows and find it far easier for insurance, finding judges, what tests/classes to use, etc..
If you're going to run an un-licensed show then make sure you are in contact with an insurance company, or the school/facility to make sure you have adequate insurance for your event. Then your next thing is to find officials who are not affiliated with an organization that requires sanctioned shows. And then as far as classes, make sure you are not using any classes/patterns that require sanctioning to use.
Then get a date, make a prize list, etc. Include in your entry regulations any barn rules, if stabling is available, where and what kind of footing the show will be held on, dress regulations, helmet requirements and membership requirements along with any other rules you wish to have. And how prizes will be distributed and what your closing date for entries are, and what you will limit entries to.
You will have to order ribbons and prizes, numbers, clipboards, pens, etc.. and all other supplies you may need for a certain class.
They say money doesn't buy happiness -- well happiness doesn't buy horses!