Before starting a career you need to know what sort of DJ you want to be. If you're not in to music much, then it probably isn't the career for you (unless you're loud and arrogant, which would suggest you still might make a great radio DJ!) For this website I have split the DJing types into 2 main categories: Mobile/special events DJs and pub/club DJs. Both styles of DJing require that you have access to a relatively large selection of music.
If at all possible I would recommend that you find a friend or local community centre that already has some equipment that you can practice on. This way you won't have to spend lots of money on DJ equipment before you are sure you want to be a DJ. DJ equipment can be very expensive, and a new pair of decks are going to be worth hundreds of pounds less than what you bought them for as soon as you open the packaging. The main thing to focus on is buying records/CDs. DJing is an expensive hobby, but if you persevere it can make a great career.
To do any DJing there are only four essential items you really need (once you've got your music). Unless you are only playing in larger pubs or night clubs where most equipment is provided, you will definitely need an amplifier and speakers. Without this no one is going to hear your music! Often mobile DJs and pub bands will use speakers with built-in amplifiers (active speakers). This will save you the hassle of carrying a separate amplifier everywhere you play, but tends to be more expensive. The alternative approach is to use a powerful amplifier and separate speakers. When starting out, the auxiliary socket in the back of most stereos will give you a basic amplifier and speaker setup.
The third essential item is a mixer. Without this, you would have to stop the music every time you wanted to change the record or CD. There's nothing worse than large silent gaps between each song, and people are unlikely to stay on the dance floor if there's a considerable delay between each song. Some people might say that the use of the mixer is half the skill of DJing. Without this there would be no beat mixing or scratch mixing - the two main styles of swapping between songs. Even if you're not "blending" the two songs together, you ideally want to swap between songs on the same beat to keep the rhythm as much as possible and prevent the two songs from clashing.
Lastly, but by no means least, you need a (dual) source of audio. Traditionally this would consist of two variable speed record decks, usually Technics 1200/1210 (renowned for quality and durability). However, with the advent of modern technology and digital audio there are several other choices, most noticeably the variable-speed CD players. Recently there have also been dual minidisk players and even variable speed MP3 players (for a PC). There are advantages and disadvantages to these newer systems - see the equipment section for more information.
Although not strictly essential for some styles of DJing, a DJ's audio sources will invariably be variable speed. As the most popular way of mixing dance music is to alter the speed until the beats are in time, most DJs rarely use a fixed-speed devices. For further information see the equipment section
The overall cost, excluding the music, ranges from £500 for a very basic setup to around £2500 for a quality mixer, amp, speakers and two Technics record decks. It's worth remembering that you get what you pay for though. Cheap record decks or CD players might seem like value for money, but two years down the road you'll be cursing the fact that you haven't got an "instant" start when you release the record or press play on the CD player; cheap belt drive decks with low-torque motors are renowned for making scratching difficult if not impossible.
Unlike most modern music released to the public, dance music is still available on that "old" format: vinyl. This is still the DJ's preferred format. To help you build up a large collection of up-to-date music, it is important that you find one or more good, reliable record stores. Any good vinyl record store should allow you to listen to the music before you buy it. This means that you won't have to constantly read the latest "buzz" charts, magazine reviews or listen nonstop to specialist radio shows to help you pick your music. Currently, it is rarer to find stores that let you play CDs before you buy (though most large stores have the most popular tunes playing in their listening booths). A more recent place to buy your music is right here on the Internet. There are several online dance music record shops out there, with one or two allowing you to listen to 20 second samples of each tune before you place you order.
Before playing to the public, it's a very good idea to practice and refine your skills first. There is probably nothing worse than screwing up your first gig, as this is likely to dent your confidence and put you off DJing in the future. Provided you enjoy DJing and enjoy buying music, then with plenty of practice there's no reason why you can't become a successful DJ and make a living from what you enjoy doing. Even if you're not making a living from DJing, you can still do it alongside your day job, as nearly all gigs are in the evenings.