The usual error is buying a cropped t-shirt in whatever size you'd normally pick and assuming the cropping is the only thing that's changed. It isn't. Brands tend to narrow the body too, so you end up with something that's short and tight, which on most people reads as a size too small rather than a deliberate shape. The crop should end somewhere between the waistband and the navel, not riding up under the ribs every time you lift your arms.
Once you've got the length right, it's a genuinely easy piece to build around. High waisted jeans or a skirt that actually sits at your natural waist, not low slung, because the whole point of the crop is that sliver of visible waist, and if your bottoms sit low you just get a gap of midriff that looks accidental rather than chosen. It also works layered under a shirt left open, or under dungarees, where the crop stops it bunching up into a lump of fabric at the waistband the way a full length tee would. For fit, go up a size from your usual if the cut is close, and check the shoulder seam sits on the shoulder rather than halfway down the arm, which is the tell that you've gone too small chasing a cropped look. The armhole matters more than people expect too, a tight one will ride up all on its own within an hour, no help needed from movement.
If you've got none, start with a slightly boxy cotton one in a plain colour, not a fitted ribbed version. The boxy cut is more forgiving of a big lunch, less likely to roll at the hem, and it works with both a high waist and, if you're feeling it, tucked loosely rather than left to its own devices. Ribbed and fitted ones are lovely but they show every seam of what's underneath, so they're a second purchase once you know what you like, not a first one.
Cotton crops soften and shrink a touch in the first few washes, which can be no bad thing since they're often cut generously to start, but it does mean the length can creep up shorter than you bought it. Wash cold, skip the tumble dryer if you can, and it'll hold its shape through a proper season of wear rather than stretching out at the hem into something that flaps. Thrown in a bag, a cotton one shrugs off creases better than anything with a high synthetic content, which just comes out looking slept in.
Wash it inside out and it keeps its colour for noticeably longer.