So first things first: Who's going?
Obviously you are, but if you want you can bring a family member or friend along too! The first time I went horse riding, I went with a friend from school. However if you don't want to do this, it's completely fine! You'll develop heaps of friends when you horse ride- I know I have!
How?
This might not seem important, but it really is. If you have a driving license, you'll be able to drive yourself there and back. But if you don't, it's probably going to be your parents who end up taking you. You have to remember that while your having fun, it's THEIR time you're taking up. Also, who's paying for it? Horse riding is a very expensive activity so make sure you don't take it for granted that someone's paying for you to go!
What?
I suggest starting with a few private lessons to learn all the basics. Then after you can confidently walk, trot and maybe canter I suggest you start some group lessons. These are what I do and I think they're much more fun cause you get to ride with your friends. In your group lessons you can work on things like jumping, dressage and maybe cross country or bareback!
When?
I suggest going once a week, but to start off you may want to just go a few times in the holidays to make sure you like it. Horse riding isn't for everyone! But if you do enjoy horse riding, you should start going regularly so you don't forget what you've learned over time.
Where?
Check on the internet - it isn't very hard! All you have it do is type in "Horse riding in (your city/town/suburb here)". You can ask your friends for recommendations if they horse ride too. If you live in Sydney Australia, I can give you some recommendations:
Kurnell Boarding Stables and Riding School: Where I am currently riding- they have great instructors and horses.
Darkes Forest- For less serious riders, Saddle Club at Darkes Forest is great, they meet up once a month and work on different things like jumping, dressage, etc.
Otford Farm- They hold awesome camps here, where you stay and sleepover for 5 days and 4 nights. You have the same horse for the whole 5 days and you are grouped according to your ability.
Eastside Riding Academy- I have never rode here myself but I have some friends who do and they love it here. The JRC (Junior Riding Club) is great to learn theory and improve your riding.
Why?
Horse riding is enjoyable, fun and a great way to spend your time. I find it better than other sports as it's not just your ability that affects your riding, but how you understand and bond with your horse.
That's all, I guess :) I hope this helped and that you start riding soon, if you haven't already!