Good denim trousers should feel like a slightly smarter version of a favourite pair of jeans, not a different garment altogether, because the point of them is that stiffness and that straighter line, worn somewhere a jean would look too casual. If they feel exactly like your weekend jeans when you try them on, they're not doing their job.
The decent ones soften over the first few wears without losing their shape, which is the balance you're after, and the way to check for it in a shop is to scrunch a handful of fabric in your fist and see how many lines stay behind when you let go. Too many and they'll bag at the knee by October. Wash them inside out, cold, as little as you can stand, and they'll keep their colour for years rather than fading into that grey nothing that happens to denim left in a hot wash with everything else. Hang them rather than fold them over months of wear and the crease stays roughly where you put it, which matters more than it sounds once you've owned a pair that creases wherever it fancies.
They pair with almost everything that isn't also denim, which is really the whole argument for having a pair, a plain jumper and boots, a shirt tucked in, a blazer if you're making more of an effort than the trousers alone suggest. Where they struggle is anything too fitted on top in a similar weight, a leather jacket say, when the whole outfit starts to feel like one stiff block rather than an outfit.
Fit is where most people, including me for longer than I'd like to admit, buy the wrong size and blame the trousers. Sit down in them in the shop, properly sit, not perch, because a waistband that's fine standing can dig the moment you fold. Check the rise against your own proportions rather than the size label, since a mid rise on one brand sits an inch lower than another's, and look at the hem unworn, standing straight, to see where it actually falls rather than where the mirror suggests. If the waistband gaps at the back when the front fits, that's your shape being narrower there than the pattern assumes, not a fault in you.
If you own none, start with a straight leg in a mid, slightly worn looking wash rather than stiff dark indigo or anything pale, because it's the one that slots into the most outfits without announcing itself. Everything else you already like in your wardrobe looks a bit more put together standing next to a pair that fits properly.