The mistake is treating a cropped t-shirt as a normal t-shirt with a few inches lopped off the bottom. It isn't. The pattern is different through the shoulder and the sleeve, and a tee that's simply been shortened tends to pull across the chest and gape at the neck in a way a proper cropped cut never should. If a crop top looks like an afterthought rather than a design decision, that's usually exactly what it is.
Where it earns its keep is with anything high waisted. Jeans, wide leg trousers, a skirt with a proper waistband, all of them give the hem somewhere sensible to land, rather than leaving a strip of midriff on show every time you reach for a top shelf. I like one under a shirt left open, or under a cardigan with the sleeves pushed up, so you get the shape without the full commitment. For sizing, go by your usual chest measurement rather than your usual t-shirt size, because cropped styles are often cut closer through the body to stop them looking shapeless, and a size down from your normal just makes the whole thing ride up further than it already will. Raise your arms in the changing room before you buy. If the hem climbs past your waistband with your arms over your head, it will do exactly the same thing on a bus, reaching for a pole, or taking a coat off in a shop doorway, and there's no fixing that afterwards.
If you've none at all, start with a plain cotton crew neck in a single colour, nothing ribbed and nothing with a slogan across the front, because that's the one that will go under things, over things, and still look like you meant it, long after anything trend led has stopped feeling current. Cotton jersey like this wears predictably: the hem is the first part to go, curling at the edges after a few washes and losing whatever crispness it had new, and the underarms thin out before the rest of the fabric does, especially if it's been balled up at the bottom of a bag more than it's been folded. None of that makes it unwearable, it just means the fit changes slightly over time, usually for the better once it's relaxed into your shape rather than the shop's.
Wash it cold, inside out, and leave it to dry flat rather than putting it anywhere near a tumble dryer, because even a centimetre of shrinkage changes exactly where that hem sits.