The one thing to remember is that most bars and clubs won't let you DJ in their establishments until you are sixteen or in some cases eighteen but it doesn't mean you can't be ready for when that time comes.
The first thing any good DJ needs is a love and knowledge of music. You need to have your finger on the pulse with the latest music and you have to know what music works well with other music.
The first thing you need is a set of DJ decks. Now these can be expensive and if you decide after a couple of attempts it isn't for you they will just sit gathering dust so before you go out and buy, try and find a set to practice on. Ask friends, local youth groups, music groups or venues if they have a set you can practice on.
Now each club has its own set up, some use computers some use CD's but 99% of DJ's all learnt on vinyl decks. If you are not sure what vinyl is ask your parents they should know.
As DJ'ing is a popular hobby there are always people upgrading their equipment and selling their old stuff so when you are ready purchase a basic set (2 vinyl decks and a mixer) second hand.
To use the decks you need a collection of the tracks you want to mix on vinyl. Now keeping your collection updated can be expensive. You can find many DJ's hunting around charity shops, car boots and market stalls increasing their collections on the cheap
Get to know your decks and what you can get out of them. Experiment as much as much as possible.
Read magazines on DJ'ing can keep up to date on the latest technology and try and update your equipment whenever you get the chance.
Listen to DJ mix CD's and learn how the professionals do it.
Offer to DJ at school discos, friends parties etc. Once you build a reputation people may start offering to pay you for your services.
Record a mix of your work and send a copy to local venues in the hopes of getting gigs. Send your work to Radio stations and music companies as well.
Start your own website and offer some of your work as downloads. This will give you the chance of getting some international exposure.