Most devices will be supplied with free DJ mixing software, alternatively will be compatible with affordable apps via the Apple iTunes store. Once you have installed the relevant app simply connect the device and you will have access to mix your MP3s with features such as scratching, EQ, and FX.
A lot of DJ kit made for the iPad will have a built-in audio interface suitable to connect to your PA system or active studio monitors, as well as cueing via your headphones, otherwise, you can also connect your speakers via the built-in 3.5mm mini jack output directly on the iOS device.
