Tables used to be the only option. Now, nearly all divers dive with a dive computer and they should.
The computer calculates your depth, time, speed of ascent, and no-decompression limits in real time. Tables can't do that. If you change depth during a dive, the computer recalculates. A table can't.
Wrist-mount computers are what most people use at this point. They're small enough, readable underwater, and you can wear them as a watch too. Console-mount models are still around but not as many people go that way now.
Entry-level computers run about $250-400 and handle everything most divers requires. Features include depth, dive time, NDL, dive logging, and often an entry-level apnea mode. Mid-range gets you transmitter compatibility, nicer screens, and more mix compatibility.
The one thing people don't think about is conservatism settings. Certain models are tighter than others. A cautious algorithm results in reduced NDL. Liberal settings extend time but at reduced margin. Neither is wrong. It just personal preference and experience level.
Worth talking to the staff at a local dive store who dives with a few different computers before buying. Staff will have honest opinions get more info on what's good and what's just marketing. The better Cairns dive stores have buying guides and rundowns online as well