Sequin dresses used to mean one thing: stiff all over sequins on a cheap mesh base that scratched your arms and shed little discs across the floor for weeks afterwards. That has changed a lot. The newer ones use softer, stretch backed fabrics with the sequins sewn on rather than glued, so they move with you instead of fighting you, and placement has got cleverer too, with panels of plain fabric doing the shaping while the sequins sit where they actually flatter. It is a better category than it was, by a good distance.
If you are starting from nothing, go for a plain sequin dress in a single colour before you touch anything patterned or ombre, because a solid colour is easier to dress up or down and harder to get wrong under different lighting. Look for long sleeves if you can find them, since bare arms in sequins can feel exposed in a way that catches people out, and a high neckline or boat neck means you are not tugging at a gaping front all evening. On the body, check where the dress sits when you sit down, not just standing in front of a mirror, because a lot of sequin dresses that look fine standing up ride up badly once you bend your knees, and a hem that sat at the knee can end up well above it. Looking after them is mostly about patience rather than effort. Hand wash or a cool gentle cycle in a bag, never tumble dry, and hang them rather than fold them so the sequins do not crack or come loose along a fold line. Over months they soften a little and move more easily, which is a good sign, but check the thread at the seams after a few wears, since that is where sequins first start dropping off. Pair it with a plain coat rather than a statement one, flat boots if you are on your feet for hours, and minimal jewellery, because the dress is already doing the talking. A long sleeve layer underneath in a similar tone can take the edge off a low back or a slit that feels like too much.
What separates a good one from a disappointing one is mostly in the hand. Pick it up and feel whether the sequins are stitched through a proper base fabric or just a thin net, check the lining sits smoothly rather than snagging on the sequins from inside, and look at the zip, a sturdy metal one will outlast a flimsy nylon zip by years. Reach for a sequin dress for anything after dark where you want to feel dressed without overthinking the rest of the outfit.