First of all, you should pick a speciality. Before taking any actions, you need to decide whether you want to be a crowd pleaser, or a real music specialist. The one from the former category just plays hit music at the parties and other private events.
A music specialist, on the other hand, associates himself with one particular genre, and does not bother about the crowd demands.
Get suitable equipment. There is no need to buy expensive stuff. If you plan to play for a venue that already has the setup, the only thing you need is a laptop with good mixing software.
On the other hand, you need a complete setup to play at a private party. The basic requirements are two turntables, headphones, mixer, speakers, monitor, MIDI controller, audio interface, mic, and different plug-ins.
Observe successful DJs very keenly. Learn the craft, and try to be different. You will surely get help from the experts, if they feel you are serious. Observe everyone, but create your own style. Copying will never help your cause.
At first, you need to go with pre-record mixes and, after getting expertise, you can mix the beats live. Either way, your main objective should be to keep the music constant in order to avoid any pause for the dancers.
Learn about the genres of music. Always keep the crowd demands in your mind. Don’t be one-dimensional, and try to build your charisma. Always adopt a professional approach.