Greetings Ids!
First you should clarify if you want to DJ or produce. You'll need different stuff depending on which it is.
Assuming you want to produce new music: in general, you'll need synthesizers/samplers, MIDI sequencing and audio editing/mixing. There's lots of free software synthesizers, and lots of good ones that aren't free. Digital Audio Workstations (DAWs) which combine MIDI and audio are not as free. There's a couple that are free/cheap, but the more popular ones cost a bit. Cubase and Ableton Live are both quite popular, but aren't cheap.
Personally I love hardware synths, but they're not cheap either. At the least you'll probably want a MIDI controller to control software synths. Some people do without and do everything with their mouse, but I suggest a controller.