DJ
Disc Jockey
A person who chooses tracks, decides the order and the timing of switches, and delivers them to the people there. The role differs from that of the person who makes the music (the producer); a DJ expresses themselves through how they line up and connect tracks that already exist. Operating the equipment is the means of making that selection work.
What a DJ is actually doing
A DJ in the booth may look like they are just turning knobs. What their hands are actually doing is preparing the next track. While listening to the track playing in the room, they listen to only the next track in their headphones, match its speed and beat position, and decide where to bring it in.
The audience only hears the moment the two tracks swap. Behind that, the DJ repeats three things for every track: choosing the next one, matching it, and swapping it in.
Track selection and mixing
A DJ's job splits broadly into selection and mixing. Selection means watching the floor and deciding what to play next. The same track lands differently in the warm-up at the start of the night and at the peak time when the room is at its height.
Mixing means moving from one track to the next without the sound cutting out. You line up the beats with a beatmatch and swap the sound with the EQ and channel faders. However smooth the mix, the floor will not move if the track you picked does not fit the room. That is why selection comes first.
What you need to start DJing
You need equipment that can handle two tracks at once, and the track files. For practice at home, a DJ controller or an all-in-one DJ system, which puts two decks and a mixer in one unit, is the easy option. Clubs usually have two CDJs and a mixer, so getting used to that layout means you will not be lost on the night.
Organize your tracks in management software such as rekordbox, then export them to a USB stick to bring with you.