If you have any friends that are DJs, stick to them like glue. Like with any thing you want to learn in life, having someone close by that understands the entire process will be your best asset and easiest route to get to where you want to be. Tell your friends you're interested in becoming a DJ, and they'll be more than happy to help out. Observe them in action, and ask questions when you can. Teaching others the art of being a DJ is part of the fun. Remember that for the future.
No matter if you have friends that can teach you or not, you should always find some well-known DJs that you like and listen to their music and watch their technique. You may not know what exactly it is their doing, but finding someone to inspire you who has made it big will definitely keep you going when times get tough. DJing is not easy, but just remember that these professionals were in your shoes not too long ago. You can easily find DJs through www.youtube.com.
Think about what kind of music you like, and if that's the style you want to DJ. You can theoretically DJ any style of music, as you have total control of the selection of music you want, but it's going to be tough when it's country or classical. The main types that are easy to get started with are hip hop, house & trance, or "top 40" music (like you hear on the radio). Personally, I think starting with hip-hop is easiest, because it has a wide range of beat styles and tempos that will teach you what you need to know in order to master any other style. Plus, if you want to make money from being a DJ, it's much easier to find a job as a hip-hop DJ at a club, for example.
This isn't necessary, but it's fun and may give you some motivation: you should think of a DJ name. Every DJ has a name, and it can be about anything. It's up to you to decide, and should be something unique to you and your personality. Some famous DJ names are: DJ Qbert, DJ Flair, DJ Magic Mike, DJ Flash, DJ Scratch, etc.
Next, you'll need to get yourself some equipment. You can ask your friends for some help in this arena, but the usual setup for any beginner is two turntables, needles, a mixer and some speakers (and an amplifier unless you are using self-powered speakers). You can go to your local guitar center or audio store to get some good information about you should start out with. You shouldn't need anything too fancy or expensive to begin with, but also don't go for the cheapo brands, because you'll have a harder time learning on bad quality equipment. If you're lucky, you may be able to find some used ones on ebay or craigslist.
Next, you'll need to get some music to play on your turntables. If you want to go old school, you'll need to go to the local record store and buy your favorite songs. If you go this route, just buy a few records that you like, and we can worry about building your library later. If you think you may ever want to start learning how to scratch, pick up some "battle records" or "sample records", which are records that just have random sounds on them which are used for scratching. If you don't think you'll want to scratch, don't worry about those.
The latest way to get music onto your turntables, is actually digitally. Depending on the exact kind of turntables you are going to use, you can use mp3 files in a CD form, or loaded directed onto your computer (which can connect to certain turntables and mixers), which is great because you can store tons of music without having to carry around heavy crates of records.
The responsibility of a DJ is to play music and never let the music stop. The beat should be able to last forever without any breaks, and that is all done by you and your equipment. The DJ is also responsible for noticing the mood of the crowd, and being able to change songs that make everyone excited at all times. You have a lot of control over your audience and their mood during a party or at a club or wedding, so your technical skills must be precise. Any slip up, and you could get booed. This is why you'll have to practice constantly as you begin to learn the techniques.
Those are all of the steps involved in getting started. As you can see, there are a lot of things involved, but trust me - it's worth all of the effort and practice. This will be the first of a series of articles about How to DJ. Be sure to look for more articles in the resources section below as more articles get written.