If you're holding a pair of cotton trousers in the changing room and they feel stiff enough to stand up on their own, put them back. The good ones soften within a few wears, not a few washes, and you want a hint of that already in the fabric before you've even paid for them.
The difference between a pair that lasts and a pair that sags by lunchtime usually comes down to the weight and weave of the cotton rather than anything written on the label. A heavier drill or twill cotton, the kind used for chinos, holds its shape through the knee and across the seat, where lighter poplin or a thin cotton blend will bag out after one sitting at a desk. Look at the waistband construction too: a proper curved waistband with interfacing inside it sits flatter than a flimsy strip of fabric folded over and stitched, which will roll and dig in by the afternoon. Check the stitching along the inside leg and the pocket bags, since that's where cheap trousers give themselves away first, with loose threads or seams that pucker. Buttons and zips should feel solid when you do them up, not like something you'd be replacing within the year, and the hem should hang straight rather than twist when you walk a few steps in them, which tells you the grain of the fabric was cut properly.
In the shop, sit down in them before you decide anything. Cotton doesn't have much give unless there's a touch of elastane woven in, so what feels fine standing at a mirror can pull tight across the thigh the second you bend. Check the rise against your own proportions rather than the mannequin's, and look at how the pockets lie flat rather than gaping open, which happens more than it should on the cheaper end of the rail.
For everyday wear, cotton trousers do most of their work alongside simple things: a fine knit jumper tucked in, a plain cotton shirt with the sleeves rolled, trainers on a weekend or loafers for anything that needs to look slightly more put together. They're forgiving with colour too, taking navy, white or stripes just as easily as a plain jumper in a neutral shade, which makes them one of the few trouser styles that doesn't ask much of the rest of the outfit.
If you've never owned a pair, start with a straight leg in a mid weight cotton rather than anything cropped or heavily tapered, since it suits the widest range of bodies and occasions. And for a first colour, go for a soft stone or khaki rather than white or black. It hides a lot, goes with almost everything you already own, and won't show every crease by the end of the day.