Good denim trousers should be boring in the best way, the thing you reach for without thinking, and most of the pairs on the high street fail that test because they're cut for a photograph rather than a Tuesday. I bought a pair two sizes too fitted once because the mirror lighting flattered them, and they sat in a drawer for a year looking lovely and doing nothing. The right pair earns its spot by being the one you grab half asleep.
Decent denim trousers get better with wear, which is part of the appeal and part of the trap. The first few washes will have them feeling stiffer than you'd like, so wash them inside out, cold, and skip the tumble dryer if you can bear the wait, because heat is what breaks down the fibres and gives you that thin, papery knee within a few months. Worn properly they soften where your body actually bends, and a good pair will hold its shape through a winter of wear and still look like trousers rather than leggings by spring. Check the seams and the pocket bags before you buy, run a thumb along the inside leg seam, if it feels thin or the stitching looks sparse, that's where the hole starts.
As for what they go with, that's the easy part, which is exactly why they're worth getting right. A plain jumper and boots, a shirt tucked in loosely, even a blazer if you want to look like you've made an effort without actually making one. They sit underneath most things in a normal wardrobe rather than competing with them, which sounds like faint praise until you own a pair that does nothing but clash.
Fit is where most people come unstuck, and it's rarely about the waist, it's the rise and the hip. Sit down in the changing room, properly sit, not a polite perch, because that's where a low rise gapes at the back and a stiff denim shows its true colours. Walk a few steps too. If the fabric has no give at all, size up rather than down, since true denim with little stretch will not forgive you the way a jersey trouser will.
If you've none at all, start with a straight leg in a mid wash, nothing too pale, nothing too dark, because it's the one that won't fight with anything else you own and won't date in two years.
Put them under a jumper you already love and the whole outfit stops looking like you tried.