What else do I need to start DJing with the DDJ-200?

It depends on how you use it.

A. When you don't use the headphones monitor
You'll need the following.
- PC/Mac or smartphone
- (When using a smartphone) USB power adapter or mobile battery (*1)

B. When you use the headphones monitor
In addition to A, you'll need the following.
- Headphones (2)
- Output devices such as power speakers and a Hi-Fi system (2)
- Cables (3) compatible with the input terminal of the device
- (For smartphones without headphones output) headphones conversion cable (4)

*1 Use a USB power adapter or mobile battery with DC +5 V and 500 mA or more.
Use a mobile battery which does not have a power-saving function.
*2 When this unit is connected via Bluetooth, the sound will be delayed. We recommend to use the supplied split cable for the connection.
*3 For example, if the input terminal of the device is an RCA pin plug (red/white),
you'll need a 3.5 φ stereo mini plug-RCA pin plug cable.
　　　Similarly, if the input terminal of the device is a 3.5 φ stereo mini plug,
　　　you'll need a 3.5 φ stereo mini plug cable.
*4 For an iPhone without a headphones output, you'll need a Lightning - 3.5 mm headphones jack adapter.