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, 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.
