There is a lot to know.
I would suggest going to your local library and picking up just about every factual book on horses they have and reading them. Do some research on the web.
You might want to get some training videos. People seem to be really into Parelli, and a lot of other natural horsemanship trainers. Read and study them. You’ll want to find a local barn/stable that gives riding lessons and take some. Also, volunteer a few days at the barn so you can see and experience first had what it’s like to own and deal with horses on a daily basis. They can also show you how to properly groom, since it’s a little hard to explain.
You’ll want to have confidence and the ability to work with a horse on the ground and in the saddle before you own one. You’ll want to know how to tack up properly as well. As for tack and such, you won’t know until you have your horse. Saddles, bridles, bits, all need to fit correctly. Along with cinches/girths and the other random horse gear.
I would really find a local barn/stable and familiarize yourself with horses and the day to day handling of them before buying a horse. They are a lot of work, time and money. And if they aren’t trained or treated right it can have really bad results which could, in worse case scenario, lead to your death.
On another note, a horse shouldn't be something you just own for 5 years and then discard. Unless you're leasing. A horse should be a lifetime commitment, though I do under selling for the various reasons, I don't understand the set of a time limit on own a horse. I'm not trying to be snipey, I just don't get it.