A good pair of mid rise jeans has a bit of structure to the waistband before you've even done the button up. Not stiff, but solid, enough that it doesn't collapse when you pick it up by one belt loop. The denim should have a bit of weight to it too. If it feels like a cheap bedsheet in your hands, it's going to bag at the knee by lunchtime.
These sit just below the belly button, which is the whole point of them, and that's what makes them so easy to build around. A plain top tucked in half, a jumper left loose over the top, a shirt knotted so it skims rather than bunches, all of it works because the rise does enough on its own without a top fighting for attention. The fit to watch is the gap at the back waistband, the bit that happens when your waist and hips aren't quite the same ratio as whoever drew up the pattern. Stand sideways in the mirror and check nothing's pulling away from your back when you sit down, because that's the gap that lets cold air in on a bus and lets everyone know you bought the wrong size. Also check the rise doesn't press right under the ribs when you sit, that's a sign you've gone a size too small and compensated by buying a high rise by accident. The other one is the thigh. Mid rise jeans often get the waist right and then go baggy through the thigh to compensate, which looks fine standing still and looks like you're wearing someone else's trousers the second you walk. Sit down in them in the changing room, properly sit, not a token bend, because that's where a lot of mid rise jeans show their hand.
If you've got none of these and need one pair to start with, go for a straight leg in a mid blue wash with a bit of stretch rather than none, no stretch sounds virtuous but it just means a worse fit by the afternoon. Skip anything heavily distressed to start, it dates faster and shows wear in places you didn't choose. A dark, uncomplicated wash earns you the most outfits for the least thought.
They do change with wear, in a good way mostly. The waistband softens and stops digging in within a couple of washes, the knees relax slightly with wear rather than stretching out of shape, and a decent pair gets better for the first few months before it eventually goes thin at the inner thigh, which is the sign you've had your money's worth and not a moment before.