A good shirt sits flat across the back and skims the chest without pulling at the buttons, so from across a room it reads crisp rather than strained. A poor one gapes between the top two buttons the second you sit down, or the collar stands away from your neck like it's not quite sure it wants to be there.
Shirts are not the answer to everything, whatever the racks suggest. A stiff cotton shirt buttoned to the top is a hard, almost severe shape, and on a soft, curvier frame it can sit like cardboard rather than cloth, especially through the hip if it's cut slim. If you need actual warmth, a shirt is doing nothing for you alone, it wants a jumper or coat over the top, and on a proper cold day it will let you down on its own. Satin ones are the worst offenders for showing every bit of what's underneath, so skip them unless you're wearing the right base layer or don't mind the outline. As for ageing, cotton and linen shirts earn their keep the longer you have them. Cotton softens with washing, loses that showroom stiffness, and starts to drape instead of stand, which I actually prefer. Linen creases on contact with a chair and that's meant to happen, not a flaw to fight, just hang it damp and most of the crumple drops out on its own. Wash on a cool setting, iron while slightly damp if you're bothered about collars, and keep satin away from the tumble dryer altogether or it goes dull and a bit sad looking.
In terms of when to reach for one, cotton and linen shirts are made for the warmer months, worn open over a vest top or knotted at the waist, and they carry a lot of weekend wear without complaint. A crisp white or pale blue one is the obvious choice for anything that needs you to look a bit more put together without trying too hard, desk days, meetings, lunches where you can't quite work out the dress code. Oversized denim or cotton shirts earn their place in the colder months too, worn as a layer over a long sleeve top, sleeves pushed up or buttoned at the cuff depending on how cold it actually is.
On colour, white is the hardest to keep looking good and the easiest to ruin with one red wine mishap, so if that's a worry, go for a pale blue or a soft stone shade instead, both hide the ordinary marks of wear far better. Black is the one that goes with almost anything you already own, which is reason enough on its own.