There are so many options when it comes to riding. In the beginning, almost any discipline will teach you the basics, but you may want to narrow it down to either English or Western riding. English riding is geared to jumping, hacking, and trails while western is about big open spaces and often cattle. The tack is different and so are the techniques used to cue the horses. It's easy to get confused if you switch between them in the early days, so pick one and find a trainer that you enjoy working with.
Don't buy a horse right away - instead take lessons and get familiar with all of the things that a horse owner needs to learn. Your lessons will start with how to safely catch and halter a horse, lead them to the barn and groom them. Learning proper techniques will keep you safe. Next you'll learn how to toss that big saddle up and do up the buckles to hold it securely (tacking up). There's plenty to learn as each horse is unique and forces us to refine our techniques to suit them specifically.
Once you are comfortable in the saddle at lessons you can start thinking about your own horse. Remember the purchase price is just the beginning. Horses are expensive, requiring good food, clean water, a safe place to rest and regular exercise. There are lots of stables where you can board or perhaps you have room for a horse at home. Make sure you check your fences and all the areas of the pasture for sharp objects, barbed wire and other hazards. Horses are masters at discovering new ways to hurt themselves!
Your trainer will help you find a suitable horse when you are ready. Although you may have thoughts about colour and size, let your coach guide your decision. It is most important that your new horse be safe and suited to your abilities. All horses are wonderful, unique individuals. Even if you aren't sure about the 18 year old, plain sorrel gelding, give them a good try. You can learn a lot on an older horse that knows all the moves you need to learn. They are less likely to spook at a flying leaf and toss you to the ground!
Riding tends to be a passion that is hard to drop, so plan ahead and allocate some regular time to your new activity. It takes many hours in the saddle to get the feel of riding and even more to start to feel confident in the maneuvers. The sooner you start, the better!