Years ago, dive tables were the only option. Now, the majority of scuba divers wear a dive computer and it makes sense.
Your computer calculates your depth, bottom time, ascent rate, and no-deco limits in real time. Dive tables are a fixed calculation. If you change depth partway through, a computer adjusts. A table can't.
Watch-style computers are the most common buy these days. These are compact, readable underwater, and you'll wear them as a watch as well. Console models are an option but less people pick them now.
Basic computers run about $300-odd and cover everything the average diver needs. You get depth, bottom look here time, NDL, log function, and usually a simple freediving mode. The $500-800 range includes wireless air monitoring, improved screens, and extra mix compatibility.
Something new divers don't think about is conservatism settings. Some algorithms are tighter than others. A tighter computer results in less NDL. Liberal ones extend time but with less margin. Both work. It's what you're comfortable with and how experienced you are.
Talk to the staff at a local dive store who's used a few different models first. Staff will offer honest opinions on what's good and what's hype. Most good dive stores publish product guides and comparisons on their websites as well