The DJs choice of music is very important. Most DJs choose a specific genre; some are versatile and choose to play all types of music. If you choose a specific genre, you must know this music inside out, from the classics to the current.
Your cd or vinyl collection must be kept in a pristine condition because you will need to use it all the time. Have backups of all your originals. Categorize your music according to the genres e.g. R nB, hip-hop, house etc.
A good DJ is always complimented about the music he/she plays. (Always remember that).