A fit and flare is the one dress shape that works with a body rather than against it, because it nips in where you're smallest and then just lets go, no clinging through the stomach or hips, no sitting there wondering if you should have gone up a size. It's the shape that looks considered even when you've thrown it on in four minutes flat, which on a weekday morning is most of what I'm asking from any dress.
If you've got none in your wardrobe, start with one in a mid weight jersey or ponte rather than anything floaty and thin, because the structure does more of the work and it holds its shape through a full day without needing adjusting every time you sit down. A defined waist seam or a wide band matters more than people admit when they're trying this shape for the first time, it's what stops the whole dress reading as shapeless rather than fitted and flared. For colour, go for something solid before you go near a print, a navy or a deep berry or bottle green will get worn on far more occasions than a busy pattern that only suits one mood. Block colour also makes it easier to tell, in the shop mirror, whether the flare is sitting properly or just billowing. Check the waist seam sits where your actual waist is and not two inches higher or lower, check the skirt has enough flare to move but not so much it feels like a costume, and check the sleeve if there is one, because a fitted bodice with a tight sleeve is a very different dress to live in than one with room to lift your arms over your head.
This is a dress for the colder months more than the height of summer, honestly, a good wool mix or ponte version sits well under a coat and still looks finished with bare legs or tights, where a thin summer version can feel like it needs a cardigan the second there's any breeze. Pair it with ankle boots and a belted coat for most of the fit and flare dresses year, or flat loafers and a denim jacket if you want it to feel less done up. A longer length suits a knee high boot in winter, a shorter one works with trainers if the fabric has enough structure to not look sloppy against something casual. Keep accessories simple, the shape itself is doing enough, and you don't want a busy necklace fighting with a skirt that's already got movement and volume built into it.