The standard club setup
The professional baseline is industry-standard media players (CDJs) or turntables plus a club mixer, on a stable surface, with booth monitoring the DJ can control. Most clubs and many venues have this installed. The DJ brings headphones and music on USB; laptops and controllers appear where the venue has no installed rig.
The lines that prevent disasters
Booth monitor the DJ controls — without it, mixing is guesswork and the floor hears the errors. Stable, level surface away from foot traffic — one knocked table ends a night. Power on its own feed where possible — generators and dodgy daisy-chains cause more failures than any equipment does. A named technical contact at the venue — five minutes with the right person at soundcheck beats an hour of guessing at 11pm.
For weddings, corporate and dry venues
Venues without an installed system need sound bringing in — from the DJ, a production company or the venue's preferred supplier. The questions to settle early: who provides the PA and decks, when is load-in and soundcheck, where does power come from, and are there noise limiters (ask this one explicitly — limiters that cut power mid-set ruin more wedding floors than any other single thing).