Denim is a casual material, so you'll want to pair it with something comparable. Even if you're trying to create a classy look, pair denim with something that's "casually dressy" like chinos. It just looks out of place next to something too fancy.
Because denim is considered a neutral color itself, it works well with other neutrals. A simple black, white, or gray shirt underneath is all you need for a simple, low-key style.
Any bright, bold color can really set off denim. For dark denim, pick a color that really pops out: a bright red or orange, for example.
Use earth tones in similar but varying shades with a brown denim jacket. Make sure to add some contrast between shades. Pair a dark brown jacket with light tan pants.
The buttons should be able to close without the fabric noticeably stretching. The shoulder seams line up with your own shoulders. The sleeves should extend to the bottom of your wrist. If they're any longer, cuff them. And the bottom of the jacket should fall at the top of your own waist.
Denim is an evergreen wardrobe piece. It's a classic that goes well with many looks and styles. But it's a casual statement, so pair it with like-minded items for a cohesive style. Denim is considered a neutral color, so arrange an outfit of neutral colors. Or, add one bold statement color that stands out.