Decide how you will use the computer. Will it play MP3 files only? Will only one music file be sent by the computer to the mixer or will multiple songs be sent from the computer and "mixed" together into one seamless song? These decisions dictate whether to use a simple piece of software, such as iTunes or Windows Media Player, to send a single song to the mixer, or whether you must invest in DJ software to provide more functionality.
Invest in a professional-quality sound card. Unless you planned ahead, the sound card that came with your computer is not of high enough quality to send music to a mixer. What sounds good running through computer speakers usually becomes distorted when played on a club sound system. Purchase a high-quality sound card that can be installed on your computer, or purchase an external card that connects using a USB or Firewire port.
Purchase the correct cables and adapters. Whether you use an internal or external sound card, you need to send the music signal to the DJ mixer. Examine the outputs of the sound card as well as the inputs on the mixer to determine what cables are required. It is possible that you will need to purchase cables that have a single 3.5mm adapter leading from the sound card and an pair of RCA adapters to attach to the mixer. many external. There are many other input and output specifications, so determine what your DJ equipment requires.
Set the inputs and outputs using your sound card software. If using a sound card that sends more than one song to the mixer, you need to tell the card's software how to split the signals into different tracks. These tracks are then sent to the mixer and appear as separate channels. The software that comes with your sound card allows you to adjust these settings easily.