There are so many things to say about how a suit fits.
Here’s one, the simplest. Pretty much everything can be altered in a suit except the shoulders. Obviously the jacket cannot be lengthened; but the sleeves can be lengthened or shortened, the waist taken in or let out (both trouser and jacket), the crotch taken out/in and the trouser legs lengthened/shortened.
So when you try on a ready-to-wear suit in the sales, look at the back of your neck (in a mirror) and the shoulders. The back of the suit should neither stand away from your neck, or wrinkle up and create a little ridge behind the lapel. The first shows the cloth has too much slack, the second that it has too little. Equally, the shoulder of the jacket should go straight out and not dip; and your shoulder should not be visible pushing at the cloth of the sleeve. These are signs that the suit is too big and too small, respectively.
Another good thing to look at is the cloth across your back. Check for lines of tautness (probably across the shoulders) or slack under the arms (indicating drooping shoulders). Then take it to a good tailor to get anything else altered.
(Whilst I learnt these tips by reading, I recommend trying on several suits to make sure you can spot the signs. Even try on your existing suits to see what you can spot, before you go shopping. I didn’t really know what much of this meant until I’d seen it for myself.)