Purchase and download a DJ program such as Traktor or Virtual DJ. Do not acquire DJ software illegally by downloading it for free. Once the program is on your computer, transfer all of your digital music onto it. If you do not yet have digital music on your computer, either transfer it by ripping CDs or downloading any desired songs for your set. Again, do not download songs for free; you must pay for all the digital music you use in your DJ set.
Practice playing songs on the software before you get a paid gig. DJ software allows speed changes, syncing capabilities and other effects that can be accomplished on physical DJ hardware. It may take hours of practice before you're able to confidently mix beats together on DJ software, especially if you have multiple songs to experiment with. It helps to look at the BPM (beats per minute) of each song and sync up those songs' beats if they have a similar BPM.
Use a mouse instead of a trackpad. A mouse will enable you to have a free hand in case you need to use it to turn up the volume or control any other knob while you use the mouse to control the software on the computer screen. Remember that you will need to plug your computer into a louder speaker system if you really want to sound like a legitimate DJ. When playing an actual gig, make sure your computer is plugged in and fully charged. Also try to upgrade your computer to 2GB of RAM if you don't have it already (DJ software runs much more smoothly if you have more memory on your computer).