Before you buy anything, turn the dress inside out and look at the label, because fabric decides whether you'll actually wear this thing past June or shove it to the back of the rail after one sweaty bus ride. Natural fibres breathe. A lot of the cheap polyester and viscose blends on the high street don't, and you'll know by lunchtime, stuck to the back of your legs, wishing you'd worn literally anything else.
If you've got none of these and need one that earns its place through sheer repetition, start plain. A simple shirt dress or a loose shift in woven cotton or linen, midi length, nothing with a built in belt that digs or a zip that catches. I bought a linen one years ago thinking the creases were a design flaw and spent a whole summer ironing it before a friend told me that's just what linen does, you wear the creases, you don't fight them. Once you've got that basic shape sorted, the next one can be a wrap dress in a light cotton jersey, which forgives a big lunch and a long afternoon in a way woven fabric never will. For colours, go for something with a bit of depth rather than the palest washed out prints that look lovely on a hanger and go see through the second you step outside, I learned that one the hard way standing in a car park with the sun behind me and no cardigan to hand. Navy, a deep rust, a proper green rather than mint, these all hide a multitude of sins, including sweat patches and the fact that white cotton dresses need a slip underneath whether the label mentions it or not.
What it pairs with is the easy part, which is exactly why it gets so much wear. Flat sandals and bare legs on the actual hot days, trainers and a denim jacket when the weather can't decide what it's doing, which in this country is most of the summer. A cardigan in a thin cotton knit takes it from garden to evening without you needing to think about it, and a pair of plain gold hoops does more lifting than any amount of extra jewellery ever will. I've worn the same cotton midi to a barbecue, a work thing and a long train journey in one fortnight, and the only thing that changed each time was the shoes.
This suits anyone who wants to get dressed in under five minutes on a hot morning and still look like they thought about it.