Snowprint-
Making a business out of ranching on Sims is always very enjoyable. All you need is your created sim(s) and at least two horses, a male and female. If you are already playing, you can adopt horses using your phone, the newspaper, the computer, or by clicking on the Equestrian Centre and then clicking on 'buy horse'.
Next, you want to purchase a very large property. It has to be quite large because horses have very finicky exercise bars and they often need to run. You can build your sims a little house, and then build a large pasture with the fence tool in build mode. Remember to add a gate, and if you don't want your horses getting out, in live mode click on it and you are able to lock it so pets cannot get through. A barn is optional. Then, simply fill the pasture with horse related items from buy mode. Add a water trough so they can drink, hay bales (you'll have to scatter them in live mode, by the way), a salt lick, a ball toy, perhaps even some of the smaller jumps. You don't have to add any training activities, however, for there are training grounds as long as you aren't in a town like Bridgeport. If you aren't in Appaloosa Plains, I suggest going into edit town and making sure you have the Equestrian Center and a Training Facility (there are two).
After you have your setup, go for a ride, learn some riding skill and have your horses start working on their jumping and racing skills. If you want to breed them right away, you are going to need a stall, which is found in buy mode with the other horse objects. Get their relationship up, and the option to try for foal should appear under a romantic option. You might run into some pathing issues, but keep on trying until they get it done. Hopefully you'll hear chimes and a foal will be born in a couple of days. Another option to breed is to ride your mare into the Equestrian Centre and breed her with a stallion there.
Using the Equestrian Centre, you can enter your horse in a variety of competitions at different levels of skill after 5:00 p.m game time. Make sure you have the skill to win, and you can get a nice amount of cash and a cool trophy.
If that foal has grown up or you want to part with a hoofed friend, then just mount the horse in front of the Equestrian Centre and then click on the building. 'Sell Horse' will be an option, and depending on how skilled the horse is you get paid more for selling.
Ranching is one of my favourite things to do in sims and I hope that you enjoy it as well. I also recommend going to the first page of this thread and follow what MaryMarsh has done, for it's very informational if you want to turn horses into a profitable business. Good luck!
~Estella.